[BUGFIX] Add missing quotes to image processing suggestion

Install tool suggests to set
`$GLOBALS['TYPO3_CONF_VARS']['GFX']['processor_colorspace'] = sRGB`
if images appear too dark. In general this is the correct
suggestion, but the value must be quoted.

This change adds the missing single quotes for the value.

Resolves: #103124
Releases: main, 12.4
Change-Id: I2e1fd2ebebf1d67870145df29ec0e40b330a5065
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82975


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Bartsch <bo@cedev.de>