Update Spanish translations with a few orthographic corrections

[dverbru]

#325

[rugk]

Hi @dverbru, first of all, thanks for your first contribution to this project! :tada: :+1: 🏅 I hope you'll like this project and enjoy hacking on it… :smiley:

Please subscribe to #138 to get notified for new updates on translations etc. :smiley:

BTW you can (automatically) let issues close when a PR is merged by adding some "magic" text to your PR body. (Manually linked it now.)

Also notice there are more things to translate like the add-on listings on AMO (addons.mozilla.org) etc. See the contributing guide for more information.

[dverbru]

Added myself to CONTRIBUTORS file, thanks. Anything else?