Update the description about `mrb_load_string(); fix #65

Author: matz

docs/articles/executing-ruby-code-with-mruby.md

@@ -78,7 +78,8 @@ main(void)
 {
   mrb_state *mrb = mrb_open();
   if (!mrb) { /* handle error */ }
-  // mrb_load_nstring() for strings without null terminator or with known length
+  // mrb_load_string(mrb, str) to load from NULL terminated strings
+  // mrb_load_nstring(mrb, str, len) for strings without null terminator or with known length
   mrb_load_string(mrb, "puts 'hello world'");
   mrb_close(mrb);
   return 0;