[DOCS] Switch extensionname and pluginname in ext:form

These two parameters are switched.

Releases: master, 10.4, 9.5
Resolves: #92887
Change-Id: I7a896a167f39b8a68664ef6f37b3b1be447825c9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/66712


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>