Swap order of MatchConditions alternatives

In our PEG-style grammar, the order matters, and in this case, we need
`MatchGuardChain` to come after `Expression`.

Thanks-to: Eric Huss <eric@huss.org>

Author: traviscross

src/expressions/match-expr.md

@@ -20,8 +20,8 @@ MatchArm -> OuterAttribute* Pattern MatchArmGuard?
 MatchArmGuard -> `if` MatchConditions
 
 MatchConditions ->
-     Expression
-   | MatchGuardChain
+     MatchGuardChain
+   | Expression
 
 MatchGuardChain -> MatchGuardCondition ( `&&` MatchGuardCondition )*