fix TemplateMixin grammar

Currently, following syntax are allowed for mixin declaration.
But the grammar does not explain it.

(From comment for `Parser::parseMixin` in `dmd/src/parse.c`)
```cpp
 *      mixin Foo;
 *      mixin Foo!(args);
 *      mixin a.b.c!(args).Foo!(args);
 *      mixin Foo!(args) identifier;
 *      mixin typeof(expr).identifier!(args);
```

This is just a reflection of current implementation behavior to the documentation.