Add missing word.
Merge pull request #13408 from Quy/master

Add missing word.
Fix incorrect offset from top when affixed element is affix-bottom.
Separate determining the affix state into a separate, expanded, method.
Always reposition an affix-bottom to better handle dynamic document height.
Add additional case for a bottom affix that has no top offset to ignore unpin.
Use double equals for null/undefined checks.