vm_args.c: make the keyword deprecation message helpful

```
$ ./miniruby -e 'def foo(kw: 1); end; h = {kw: 1}; foo(h)'
-e:1: warning: The last argument is used as the keyword parameter
-e:1: warning: for `foo' defined here; maybe ** should be added to the call?
```

vm_args.c

@@ -693,18 +693,18 @@ rb_warn_last_hash_to_keyword(rb_execution_context_t * const ec, struct rb_callin
     name = rb_id2str(ci->mid);
     loc = rb_iseq_location(iseq);
     if (NIL_P(loc)) {
-        rb_warn("The last argument for `%"PRIsVALUE"' is used as the keyword parameter",
+        rb_warn("The last argument for `%"PRIsVALUE"' is used as the keyword parameter; maybe ** should be added to the call?",
                 name);
     }
     else {
         rb_warn("The last argument is used as the keyword parameter");
         if (calling->recv != Qundef) {
             rb_compile_warn(RSTRING_PTR(RARRAY_AREF(loc, 0)), FIX2INT(RARRAY_AREF(loc, 1)),
-                            "for `%"PRIsVALUE"' defined here", name);
+                            "for `%"PRIsVALUE"' defined here; maybe ** should be added to the call?", name);
         }
         else {
             rb_compile_warn(RSTRING_PTR(RARRAY_AREF(loc, 0)), FIX2INT(RARRAY_AREF(loc, 1)),
-                            "for method defined here");
+                            "for method defined here; maybe ** should be added to the call?");
         }
     }
 }