Fix stray backslash in debian/tests/help

Santiago R.R.requested to merge
santiago/xautolock:xautolock-fix-stray-backslash into master

That stray backslash causes autopkgtest to fail when running with grep 3.8-2: https://ci.debian.net/data/autopkgtest/unstable/amd64/x/xautolock/26130620/log.gz

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