Fixes file reference in examples

[mbrukner]

Doxygen would complain about not being able to find test-integer-math.c. I suppose the file was renamed at some point, without adjusting the respective comment.

Checklist

Thanks a lot for your contribution! Please tick off the following:

• Tests run successfully (i.e. make test)
  • [x] using the CC65 compiler
  • [x] using the LLVM/Clang compiler
• [x] Doxygen style tags are used for public API
• [x] Source code is properly formatted; run e.g. clang-format -i <file>
• [x] I agree to the License of this repository
• [x] I also agree to the Apache 2 license