Add literal semicolon to the grammar of view_item

Both external crate declarations and use declarations need to end with a
semicolon.

Author: carols10cents

src/doc/grammar.md

@@ -322,7 +322,7 @@ mod : [ view_item | item ] * ;
 #### View items
 
 ```antlr
-view_item : extern_crate_decl | use_decl ;
+view_item : extern_crate_decl | use_decl ';' ;
 ```
 
 ##### Extern crate declarations