Correct grammar

Jekyll is the third person singular (it), so it should be 'its documentation', not 'their documentation'.
Merge pull request #10932 from ZDroid/patch-1

Correct grammar
Fix #10948: Clarify navbar example target in docs
Change all navbar example targets

According to comment on https://github.com/twbs/bootstrap/issues/10948#issuecomment-25751889 .
Merge pull request #10950 from twbs/docs-navbar-class

Fix #10948: Clarify navbar example target in docs