Updates the HowToAddANewAssertion.md file to be in line with Cyrille's refactoring of the ExtensibilityHelper type.
Updates the HowToAddANewAssertion.md file to be in line with Cyrille's refactoring of the ExtensibilityHelper type.

dupdob/NFluent,tpierrain/NFluent,tpierrain/NFluent,dupdob/NFluent,NFluent/NFluent,dupdob/NFluent,NFluent/NFluent,tpierrain/NFluent