offlineimap.conf: use raw strings in regex examples

[lucrocha]

Newer pythons are strict on the escape sequences allowed in normal strings, which can cause issues with regular expressions as most of them do include escape sequences that are not allowed. In those cases, the process will exit with the confusing warning: <string>:1: SyntaxWarning: invalid escape sequence '\.'

To help prevent such cases, have the sample configuration use raw strings when using regular expressions.

This PR

Add character x [x].

• [x] I've read the DCO.
• [x] I've read the Coding Guidelines
• [x] The relevant informations about the changes stands in the commit message, not here in the message of the pull request.
• [x] Code changes follow the style of the files they change.
• [x] Code is tested (provide details).

Additional information

Tested with my own configuration which uses regular expressions with \. escaped.

[etbuira]

Thanks, applied in https://github.com/IMAPMirror/imapmirror