PEP 544: Rename confusing identifiers in example

Author: aphedges

pep-0544.txt

@@ -1232,11 +1232,11 @@ Another potentially problematic case is assignment of attributes
 *after* instantiation::
 
   class P(Protocol):
-      x: int
+      y: int
 
   class C:
       def initialize(self) -> None:
-          self.x = 0
+          self.y = 0
 
   c = C()
   isinstance(c, P)  # False