Rename variable for clarity
Rename variable for clarity

This class accepts both instances of a `TaggingSpreadsheet` and a
`TagMigration`. These are called `tagging_sources` in other parts of the
codebase.

This commit changes the naming so it's clearer what that variable is referring
to.

alphagov/content-tagger,alphagov/content-tagger,alphagov/content-tagger