fixed syntax

Author: Kivooeo

src/expressions/match-expr.md

@@ -21,24 +21,7 @@ MatchArm ->
 
 MatchArmGuard ->
       `if` Expression
-    | `if` LetChain
-
-LetChain ->
-    LetChainCondition ( `&&` LetChainCondition )*
-
-LetChainCondition ->
-      Expression _except_ [ExcludedConditions]
-    | OuterAttribute* `let` Pattern `=` Expression _except_ [ExcludedConditions]
-
-ExcludedConditions ->
-    IfExpression
-  | MatchExpression
-  | WhileExpression
-  | ForExpression
-  | LoopExpression
-  | AsyncExpression
-  | TryExpression
-  | AwaitExpression
+    | `if` Expression && LetChain
 ```
 <!-- TODO: The exception above isn't accurate, see https://github.com/rust-lang/reference/issues/569 -->