[BUGFIX] Add missing backslashes to class name in rst file

Namespace and classname have not been separated by a backslash,
making that class not easy to identify.

Resolves: #79341
Releases: master, 8.7
Change-Id: Ic86086524cb10c470296facef25759f14c35cc11
Reviewed-on: https://review.typo3.org/54960


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>