Tutorial: make minor fixes

This PR makes two three minor fixes to the Tutorial:

• Add missing $ to a generator expression

• Removes redundant PROJECT_BINARY_DIR in include. Rationale: The CMakeLists.txt in Step2 includes the PROJECT_BINARY_DIR. The PROJECT_BINARY_DIR in the solution unrelated to the TODO 4 ("Add MathFunctions to Tutorial's target_include_directories()").

• Add "the existing" to clarify the expectation is to modify the existing target_include_directories, not add a new target_include_directories command.

• Updates the INSTALL_DESTINATION in Step11. I expect the reference to example is intended to be MathFunctions to match the DESTINATION in the target(INSTALL) below.