update en-GB translation
update en-GB translation

Change-Id: I81e04ead1dbe2179fa7f00de94a6af9fc260733f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/217502
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

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