Fix typo of expanded icon class

[0xSbock]

The css class name is misspelled. Thus, the chevron-down is not rendered.

Class definition in redmine:
https://github.com/redmine/redmine/blob/master/public/stylesheets/application.css#L1668

[mrliptontea]

The typo came from the source when these icon classes were introduced - https://github.com/redmine/redmine/blob/4.1.0/public/stylesheets/application.css#L1530

So for backward-compatibility they will need to be duplicated in PurpleMine. Can I ask that you add the fix as a new line and leave the old, and then build the project and include compiled application.css in the Pull Request - I could then add a new release with a fix.

[0xSbock]

Hey :) thanks for the quick response!
Sorry i didn't catch that this could lead to backward compatibility issues.
I force-pushed a new version of the fix to my branch which includes the fix and the typo, as well as a comment.

I wasn't sure if i should include the application.css in the same commit. If you want me split it up i can do this tomorrow :)