docs: update docker-compose documentation url in configuration.md

[AnzhiZhang]

Description

Change the docker-compose documentation url to the correct one. It should be Merge Compose files instead of extend.

See also https://github.com/overleaf/toolkit/blob/7f10105a9be889d29e3f30a7e7733f678f8c1002/bin/docker-compose#L54-L57

Related issues / Pull Requests

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