update en-GB translation
update en-GB translation

Change-Id: Ib7c0314251f8e75c5a3696e80599514803ebc1ba
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/301812
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

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