data: Fix grammar and wording in new GSettings key

The description for the 'welcome-dialog-last-shown-version' key
must have been written quite late in the day, as it's full of grammar
mistakes, typos and British English.
*sips coffee*
I can say that, because I wrote it.
*sips coffee*
Anyway, that might be a bit better.

*sips coffee* and Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1641