Fixed typo in form docs
Bugfix for #3840

This fixes issue #3840 and I guess it was a typo before...
Fixed large-only and xlarge-only visibility classes
Fixed some typos in visibility scss
Merge pull request #3851 from christianseel/patch-2

Bugfix for #3840
Merge pull request #3847 from dillongrove/master

Fixed typo in form docs