This makes some small fixups to things I noticed when starting a translation of gtkmm-documentation.
- Typos (specifed etc.)
- Spacing fixes
- gtkmm4.0-devel now has a . in the name in Fedora https://packages.fedoraproject.org/pkgs/gtkmm4.0/gtkmm4.0-devel/
- Move the plural s out of some class names (it was already placed outside the tag in outer strings).
Also saw some British English spellings. If it is desirable i could make another MR to change these to American spelling.