Fix typos in no-vendor-prefixes rule documentation

[basvandenberg]

There are some typos in the documentation of the no-vendor-prefixes rule:

In the sub-section Additional Identifiers the word test should be replaced with khtml.

In subsections Additional Identifiers and Excluded Identifiers there is an example yml in which:

• The no-vendor-prefix: should be replaced with no-vendor-prefixes:.
• The key 'excluded-identifiers' should be without quotes (excluded-identifiers).
• The corresponding values ('khtml', 'webkit', 'moz') should also be without quotes.

[DanPurdy]

Thanks @basvandenberg do you have a PR waiting for this?

[basvandenberg]

Just created a new pull request.