make the error message clearer

Signed-off-by: Boik <qazbnm456@gmail.com>

docker/api/container.py

@@ -838,7 +838,8 @@ class ContainerApiMixin(object):
                         params['since'] = since
                     else:
                         raise errors.InvalidArgument(
-                            'since value should be datetime or int, not {}'.
+                            'since value should be datetime or positive int\
+                            , not {}'.
                             format(type(since))
                         )
             url = self._url("/containers/{0}/logs", container)