[BUGFIX] Wrong header in 8.6 .rst file

Deprecation should be Feature to fix the list on
https://docs.typo3.org/typo3cms/extensions/core/8-dev/Changelog/8.6/Index.html

Change-Id: Ic514b1adbb6e624ff9f740a0a9f020f848d7b0df
Resolves: #80332
Releases: master
Reviewed-on: https://review.typo3.org/52087


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>