fixed if let guards documentation

Author: Kivooeo

src/expressions/match-expr.md

@@ -18,8 +18,7 @@ MatchArms ->
 MatchArm -> OuterAttribute* Pattern MatchArmGuard?
 
 MatchArmGuard -> `if` Expression
-               | if Expression
-               | if let Pattern = Expression
+               | `if` `let` Pattern Expression
 ```
 <!-- TODO: The exception above isn't accurate, see https://github.com/rust-lang/reference/issues/569 -->