fixed docstring error

Author: arthurlw

pandas/core/groupby/groupby.py

@@ -3846,7 +3846,7 @@ def expanding(self, *args, **kwargs) -> ExpandingGroupby:
         4     B     50
         5     B     60
 
-        >>> df.groupby("Class").expanding().mean()\
+        >>> df.groupby("Class").expanding().mean()
                  Value
         Class
         A     0   10.0