[FOLLOWUP] Rename GFX settings referring to IM in their name

Replace wrong "ß" character with backslash for escaping.

Change-Id: I36188f5c1eb1a6c99b040b3e8783fb83807d5aae
Resolves: #43085
Releases: master
Reviewed-on: https://review.typo3.org/46073


Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>