[Atlas] Fix codeblock in two places to make the content display

Fix /source/includes/mongomirror-required-roles.rst
and add missing empty lines after code blocks.

https://docs.atlas.mongodb.com/import/live-import/ page has two empty sample fields.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)