Tutorial: Fix grammar typo in Step 11

Fix and improve wording in the "Adding Export Configuration" step.

Original Description

https://gitlab.kitware.com/cmake/cmake/-/blob/master/Help/guide/tutorial/Adding%20Export%20Configuration.rst?ref_type=heads. We found some minor language issues in the documentation so figured that would be a good entry point instead since we struggled with the other contribution. In the end, we found some typo grammar errors to fix.

Topis-rename: tutorial-grammar-typo