Reword mention of mrbtest in README

I'm not really sure I correctly understood the meaning of `"mrbc" is also able to generate compiled byte code in a C source file.
You can check the "mrbtest" program under the "test" directory.`

I guess mrbtest is meant as an example for using byte code in a C source file and tried to clarify that meaning.

If I'm wrong, just ignore this. :)