Fixed typo of macro in example at README.md

Hi John.
Thank you for replying :)
I made a new pull request and closed last one(https://github.com/cpputest/cpputest/pull/204).

I mistook LONGS_EQUAL for CHECK_EQUAL, and thought CHECK_EQUAL is duplicated.
I fixed typo.

Hope this helps you :thumbsup: 

Best.
Kawasaki