[BUGFIX] EXT:form - fix wrong "templateFileName" usage within the docs

Resolves: #81799
Releases: master, 8.7
Change-Id: I89a653bf58376e42a02da0fefd5bbc7879a07a45
Reviewed-on: https://review.typo3.org/53408


Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Patric Schumann <psm@fh-wedel.de>
Tested-by: Patric Schumann <psm@fh-wedel.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Henrik Elsner <helsner@dfau.de>
Tested-by: Henrik Elsner <helsner@dfau.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>