doc: escape backslash This avoids a SyntaxWarning on Python 3.8 due to invalid \ escape. Differential Revision: https://phab.mercurial-scm.org/D5843

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}