chore: escape "\." in re_split() docstring

This was giving me a `SyntaxError: invalid escape sequence '\.'`

ibis/expr/types/strings.py

@@ -1082,7 +1082,7 @@ def re_extract(
         why="Different backends support different regular expression syntax."
     )
     def re_split(self, pattern: str | StringValue) -> ir.ArrayValue:
-        """Split a string by a regular expression `pattern`.
+        r"""Split a string by a regular expression `pattern`.
 
         Parameters
         ----------