Help: Fix typo in CMakeLists.txt for CMake Tutorial Step2

The main thing to be aware of is that the solution CMakeLists.txt code shown under "TODO 7: CMakeLists.txt", "TODO 8: Click to show/hide answer", "TODO 11: Click to show/hide answer", "TODO 12: Click to show/hide answer" and "TODO 13: Click to show/hide answer" all use USE_MYMATH and not MY_MATH.

Remaining work to be done: The tutorial documentation related to the above (https://cmake.org/cmake/help/latest/guide/tutorial/Adding%20a%20Library.html) also needs to be fixed. Throughout the normal text discussion on that page, both "MY_MATH" and "USE_MYMATH" can be found. Apparently "MY_MATH" should be changed to "USE_MYMATH" there as well. (I haven't created a commit for that, but if you tell me the file in the repo to go to I'd be happy to do it).