Fix missed doc grammar rule rename from vec_elems to array_elems

Author: alancutter

src/doc/grammar.md

@@ -514,7 +514,7 @@ field_expr : expr '.' ident ;
 ### Array expressions
 
 ```antlr
-array_expr : '[' "mut" ? vec_elems? ']' ;
+array_expr : '[' "mut" ? array_elems? ']' ;
 
 array_elems : [expr [',' expr]*] | [expr ',' ".." expr] ;
 ```

src/doc/reference.md

@@ -2821,7 +2821,7 @@ automatically dereferenced to make the field access possible.
 ### Array expressions
 
 ```{.ebnf .gram}
-array_expr : '[' "mut" ? vec_elems? ']' ;
+array_expr : '[' "mut" ? array_elems? ']' ;
 
 array_elems : [expr [',' expr]*] | [expr ';' expr] ;
 ```