reference grammar: remove duplicate else_tail rule

Author: matklad

src/doc/grammar.md

@@ -688,7 +688,6 @@ match_pat : pat [ '|' pat ] * [ "if" expr ] ? ;
 ```antlr
 if_let_expr : "if" "let" pat '=' expr '{' block '}'
                else_tail ? ;
-else_tail : "else" [ if_expr | if_let_expr | '{' block '}' ] ;
 ```
 
 ### While let loops