modules.service.__virtual__: use raw string for regex

************* Module salt.modules.service
salt/modules/service.py:57: [W1401(anomalous-backslash-in-string), ] Anomalous backslash in string: '\D'. String constant might be missing an r prefix.
Merge pull request #15149 from cvrebert/patch-1

modules.service.__virtual__: use raw string for regex