fix typo in modules.file.replace() docstring

************* Module salt.modules.file
salt/modules/file.py:1020: [W1401(anomalous-backslash-in-string), ] Anomalous backslash in string: '\s'. String constant might be missing an r prefix.
Merge pull request #14146 from cvrebert/patch-1

fix typo in modules.file.replace() docstring