fixed typo in gcc flags

As I was learning how to use your C testing framework, I noticed a difference between the code for compiling and the description of that code. The example code correctly shows the proper flags (-lrt -lm) but the subsequent description show -ltr -lm (unless -ltr refers to something else that I'm not familiar with).

Otherwise, the minunit code works great for me. It was very easy to setup and use.