Fix invalid CSS ending

[hinaloe]

Proposed changes

https://github.com/awesomemotive/all-in-one-seo-pack/blob/a65d0c009cbf682a50d910fbde3e1161bd3a5e34/css/aiosp_admin.css#L440-L446

End of stylesheet, there is no closing bracket. This is probably not break some browser in simple way, but it breaks css concatting (like nginx-http-concat).

Types of changes

• Bugfix (non-breaking change which fixes an issue)

Checklist

• [ ] I've selected the appropriate branch (ie x.y rather than master).
  • looks there are no branch...
• [x] I've created an appropriate title, descriptive of what the PR does.
• [ ] If I added, removed or modified any translatable strings, I updated the commonstrings.php file (free repository) accordingly or else opened an issue for it.
• [ ] I have added tests that prove my fix is effective/my feature works or have created an issue for it (if appropriate).
• [ ] I have added necessary documentation, or have created an issue for docs (if appropriate).

Testing instructions

• Don't assume the tester knows the entire backstory of the issue, and don't force him/her to decipher the code to try and figure out what -it's doing or how to test it.
• Do provide step by step instructions on how to test.
• Do note things to watch out for.
• Do note what aspects the tester should try and break.

[wpsmort]

@hinaloe Thanks for submitting this PR. I'm closing this as we'll fix this in our other repo.