fix docstest

Author: Joan Massich

imblearn/utils/testing.py

@@ -159,8 +159,6 @@ def warns(expected_warning, match=None):
     ...     warnings.warn("this is not here", UserWarning)
     Traceback (most recent call last):
     ...
-    AssertionError: 'must be \d+$' pattern not found in ['this is not here']
-    ...
     """
     with _warns(expected_warning) as record:
         yield