Correct comment which is showing a wrong mixin call example (mixins.less)

I was copy-pasting the example and it wasn't working. Then I found that the example in the comment was a typo.
Thanks
Merge pull request #8965 from leonardovcid/patch-1

Correct comment which is showing a wrong mixin call example (mixins.less)
fix typo in CSS docs; fixes #8992

"from" => "form"
Merge remote-tracking branch 'upstream/3.0.0-wip' into 3.0.0-wip