[DOCS] Fix minor typos in form documentation

Some typos are fixed. Additionally the spelling of "customise" was
adjusted to en-us.

Resolves: #89674
Releases: master, 9.5
Change-Id: I982c8438405677b6435e38cad5f7dbb6a6e886f7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62319


Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Michael Telgkamp <michael.telgkamp@mindscreen.de>
Tested-by: Sybille Peters <sypets@gmx.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Michael Telgkamp <michael.telgkamp@mindscreen.de>
Reviewed-by: Sybille Peters <sypets@gmx.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>