Fix typo in selectors README

[VladNegara]

I noticed a typo in the README file of the selectors library.

[Loirooriol]

Since this isn't specific to Servo, can you send this directly to upstream Firefox?

If it lands here, I will need to export it anyways (which isn't a big deal if you aren't familiar with Firefox contributions)

[VladNegara]

I'm not familiar with Firefox contributions. If the process isn't too involved, could you point me in the right direction?

[Loirooriol]

See https://firefox-source-docs.mozilla.org/contributing/contribution_quickref.html for a guide.

In this case you don't need to build. Also downloading the entire history would be overkill, I think git clone --depth 1 https://github.com/mozilla-firefox/firefox/ should suffice to get the code.

With the code, just do the change, and commit. You should probably link the commit to a bug in https://bugzilla.mozilla.org/, e.g. using Bug XXXXXX - Fix typo in selectors README. r?#style as a commit title.

Then submit it according to https://firefox-source-docs.mozilla.org/contributing/contribution_quickref.html#to-submit-a-patch.

If that's too tedious, it's fine to just land it here.