Fixing link on user story training page

Why is this change being made?

This page contained a broken link to User Story Mapping caused by a missing slash in the relative URL.

This MR adds the slash back and fixes the problem.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what

@sarahj This should fix it!