Fix typos in ffap.el.

Fix typos in ffap.el.

* ffap.el (ffap-shell-prompt-regexp, ffap-all-subdirs, ffap-url-p)
  (ffap-alist, ffap-tex-path, ffap-url-at-point, ffap-gopher-regexp)
  (ffap-gopher-at-point, ffap-file-at-point, ffap-read-file-or-url)
  (ffap-read-url-internal, ffap-menu, ffap-at-mouse):
  Fix typos in docstrings.
  (ffap-url-regexp): Doc fix.
  (ffap-at-mouse): Fix typo in message.