[FOLLOWUP][BUGFIX] Broken documentation icon in extension manager

When using EXT:sphinx the documentation icon is broken since the
"btn" class attribute is not set.

Resolves: #63430
Releases: master
Change-Id: If86bd2490bb16757b3b50aa236442881c6947fef
Reviewed-on: http://review.typo3.org/34808
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>