README cmake install features

[invitus]

Make sure these boxes are checked before submitting your issue - thank you!

• [x] Ask for help on discord and/or reddit
• [ ] Consider providing a dump file using b2World::Dump

The example currently says to use: target_link_libraries(mytarget PRIVATE box2d)

which was changed from: target_link_libraries(mytarget PRIVATE box2d::box2d)

due to the issue: https://github.com/erincatto/box2d/issues/647

However, if you use the cmake install feature (as stated by the README), you have to use box2d::box2d. In the previous issue the install feature was not used, but rather a submodule with a subdirectory to the box2d cmake.

Originally posted by @invitus in https://github.com/erincatto/box2d/issues/647#issuecomment-739920762

[erincatto]

I'm just going to remove this part of the documentation because I am not testing this workflow.