Update grammar.md

Update grammar.md to reflect the changes to `if let` and `while let`.

Author: Munksgaard

src/doc/grammar.md

@@ -661,15 +661,15 @@ match_pat : pat [ '|' pat ] * [ "if" expr ] ? ;
 ### If let expressions
 
 ```antlr
-if_let_expr : "if" "let" pat '=' expr '{' block '}'
+if_let_expr : "if" "let" pat [ '|' pat ] * '=' expr '{' block '}'
                else_tail ? ;
 else_tail : "else" [ if_expr | if_let_expr | '{' block '}' ] ;
 ```
 
 ### While let loops
 
 ```antlr
-while_let_expr : "while" "let" pat '=' expr '{' block '}' ;
+while_let_expr : "while" "let" pat [ '|' pat ] * '=' expr '{' block '}' ;
 ```
 
 ### Return expressions