Fix typo in closing screenshot tag
Detected by
$ appstream-util validate-relax --nonet /builddir/build/BUILDROOT/bubblemail-0.4-1.fc32.x86_64/usr/share/metainfo/bubblemail.appdata.xml
failed to parse /builddir/build/BUILDROOT/bubblemail-0.4-1.fc32.x86_64/usr/share/metainfo/bubblemail.appdata.xml: Error on line 22 char 17: Element ?screenshots? was closed, but the currently open element is ?screenshot?