Typo and spacing fixes

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.