Fix the paper link error in the docstring (#2618)

mmcv/cnn/bricks/generalized_attention.py

@@ -14,7 +14,7 @@ class GeneralizedAttention(nn.Module):
     """GeneralizedAttention module.
 
     See 'An Empirical Study of Spatial Attention Mechanisms in Deep Networks'
-    (https://arxiv.org/abs/1711.07971) for details.
+    (https://arxiv.org/abs/1904.05873) for details.
 
     Args:
         in_channels (int): Channels of the input feature map.