filesystem: fix WARNING: Block quote ends without a blank line

Use the 'r' (raw) qualifier to avoid rendering '\n' as the actual
line break in the auto-generated documentation.

Change-Id: Ie7f59685a78534eb2c43ec4bc39685d3fd264778

pySim/filesystem.py

@@ -301,7 +301,7 @@ class CardFile:
 
     @staticmethod
     def export(as_json: bool, lchan):
-        """
+        r"""
         Export file contents in the form of commandline script. This method is meant to be overloaded by a subclass in
         case any exportable contents are present. The generated script may contain multiple command lines separated by
         line breaks ("\n"), where the last commandline shall have no line break at the end