[BUGFIX] Correct minor typo in "About TYPO3 CMS"

This patch corrects a minor typo in module: Web -> About TYPO3 CMS.
It currently reads "TYPO3 CMS is a enterprise-class...", but it should
read: "...is an enterprise..." ("an" with a "n").

Resolves: #59187
Releases: 6.2
Change-Id: I4e77e54dded08ef4cbdd484d81660a1e39241986
Reviewed-on: https://review.typo3.org/30476
Reviewed-by: Markus Klein
Tested-by: Markus Klein