"Improperly reduced borrows” example now compiles, causing its compile_fail doctest to fail

[yuzibo]

There is one issue detected by Ruyici

...
INFO Testing chapter 'Lifetimes': "lifetimes.md"                                                               
 INFO Testing chapter 'Limits of Lifetimes': "lifetime-mismatch.md"                                             
ERROR rustdoc returned an error:                                                                                
                                                                                                                
--- stdout                                                                                                      
                                                                                                                
running 1 test                                                                                                  
test lifetime-mismatch.md - Limits_of_Lifetimes (line 44) ... ignored                                           
                                                                                                                
test result: ok. 0 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out; finished in 0.00s                   
                                                                                                                
                                                                                                                
running 2 tests                                                                                                 
test lifetime-mismatch.md - Limits_of_Lifetimes (line 5) - compile fail ... ok                                  
test lifetime-mismatch.md - Limits_of_Lifetimes::Improperly_reduced_borrows (line 81) - compile fail ... FAILED 
                                                                                                                
failures:                                                                                                       
                                                                                                                
---- lifetime-mismatch.md - Limits_of_Lifetimes::Improperly_reduced_borrows (line 81) stdout ----               
Test compiled successfully, but it's marked `compile_fail`.                                                     
                                                                                                                
failures:                                                                                                       
    lifetime-mismatch.md - Limits_of_Lifetimes::Improperly_reduced_borrows (line 81)                            
                                                                                                                
test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.38s               
                                                                                                                
all doctests ran in 1.29s; merged doctests compilation took 0.91s                                               
                                                                                                                
--- stderr                                                             

It seems we need to update the document.