docs/user_guides: update repo tool install url

The previous url did not include instructions for installing the 'repo'
tool, as the instructions have been moved to a new location.

Updated the URL to the correct location.

Signed-off-by: Lokesh B V <lokesh.bv@arm.com>
Change-Id: Id9c973a9f54115c9a511faa067056515f28edd95