fixed syntax

Author: Kivooeo

src/expressions/match-expr.md

@@ -21,7 +21,7 @@ MatchArm ->
 
 MatchArmGuard ->
       `if` Expression
-    | `if` Expression && LetChain
+    | `if` Expression `&&` LetChain
 ```
 <!-- TODO: The exception above isn't accurate, see https://github.com/rust-lang/reference/issues/569 -->