Fix spellcheck failure

[jedcunningham]

Locally, building the apache-airflow docs results in a spellcheck failure:

migrations-ref.rst:151: (DBs)  Increase text size for MySQL (not relevant for other DBs’ text types)         
                                                                                                             
File path: apache-airflow/migrations-ref.rst                                                                 
Incorrect Spelling: 'DBs'                                                                                    
Line with Error: 'Increase text size for MySQL (not relevant for other DBs’ text types)'                     
Line Number: 151

I'm not exactly sure why CI passes, but this is certainly annoying to have happen locally regardless.

[jhtimmins]

@jedcunningham would it make more sense to just do DB? I assume it handles pluralization properly?

[jedcunningham]

Oh, let me try it.

[jedcunningham]

Nope, still fails.