InvariantConcepts part of the lesson 8 (possibly) missing code examples

[mmv08]

The referenced code in the lesson https://github.com/Certora/Tutorials/blob/master/08.Lesson_WorkingWithInvariants/InvariantsConcepts/README.md
doesn't match the code example https://github.com/Certora/Tutorials/blob/master/08.Lesson_WorkingWithInvariants/InvariantsConcepts/Manager/ManagerFullSolution.spec therefore making the lesson harder to follow