[BUGFIX] Update warning text in System Environment Check

* missing punctuation between "upload_max_filesize=xxx" and
"By default..."
* letter "Y" should be lowercase in: "(You can alter the
TYPO3 defaults..."

Change-Id: I1181c8d187c179db742ca439cbaca8cb531455ec
Fixes: #47656
Releases: 6.2, 6.1
Reviewed-on: https://review.typo3.org/20320
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert