formula_desc_cop: ensure no full stops at the end of desc

- This was a suggestion by Mike McQuaid in my `homebrew-core` audit
  description PR. Based on ilovezfs's incantation `grep -r -E 'desc
  ".*\."' *.rb`, some formulae descriptions do end in full stops. (My
  initial assessment of this failed to account for the fact that
  descriptions are strings and so end in `"`.)
- Add an autocorrect for this cop, too.