fix dcn doc about Mask introduction, test=develop, test=document_fix (#20836)

python/paddle/fluid/layers/nn.py

@@ -16906,9 +16906,9 @@ def deformable_conv(input,
             float32, float64.
         offset (Variable): The input coordinate offset of deformable convolution layer.
             A Tensor with type float32, float64.
-        Mask (Variable, Optional): The input mask of deformable covolution layer.
+        Mask (Variable, Optional): The input mask of deformable convolution layer.
             A Tensor with type float32, float64. It should be None when you use
-            deformable_conv_v2.
+            deformable convolution v1.
         num_filters(int): The number of filter. It is as same as the output
             image channel.
         filter_size (int|tuple): The filter size. If filter_size is a tuple,