glib-compile-schemas: Fix some minor en_GB-isms in translatable strings

Spotted by Bruce Cowan.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Fixes: #1867
14 jobs for 1867-en-gb-isms in 55 minutes and 7 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #391454
cross-android_api21_arm64

00:00:59

passed #391455
cross-android_api28_arm64

00:03:15

passed #391456
cross-mingw64

00:01:36

passed #391452
debian-stable-x86_64

00:07:01

passed #391451
fedora-x86_64

00:09:55

passed #391453
G_DISABLE_ASSERT

00:07:02

passed #399762
win32
msys2-mingw32

00:09:18

passed #391458
win32
vs2017-x64

00:10:05

failed #391762
win32
msys2-mingw32

01:00:02

failed #391681
win32
msys2-mingw32

01:00:01

failed #391588
win32
msys2-mingw32

01:00:25

failed #391457
win32
msys2-mingw32

01:00:02

 
  Coverage
passed #391459
coverage

00:02:59

77.8%
 
  Analysis
passed #391460
scan-build

00:22:48