Doxygen: Fix Typos

This MR fixes:

• Typo in documented target command for cmake doxygen
• Erroneous option in CMakeLists (which was ignored anyway)

This does not fix the PNG generation with the bash script implementation, which appeared to be present prior to @AlexBuccheri 's refactoring.

changed milestone to %15.0

added Bugfix label

requested review from @martin.lueders

assigned to @AlexBuccheri

added Documentation label

approved this merge request

merged

mentioned in commit adbfde7b

added in Changelog label