correct DepthwiseConv2D docstring (keras-team#12949)

keras/layers/convolutional.py

@@ -1546,7 +1546,7 @@ def __init__(self, filters,
 class SeparableConv2D(_SeparableConv):
     """Depthwise separable 2D convolution.
 
-    Separable convolutions consist in first performing
+    Separable convolution performs first
     a depthwise spatial convolution
     (which acts on each input channel separately)
     followed by a pointwise convolution which mixes together the resulting
@@ -1685,10 +1685,10 @@ def __init__(self, filters,
 
 
 class DepthwiseConv2D(Conv2D):
-    """Depthwise separable 2D convolution.
+    """Depthwise 2D convolution.
 
-    Depthwise Separable convolutions consists in performing
-    just the first step in a depthwise spatial convolution
+    Depthwise convolution performs
+    just the first step of a depthwise spatial convolution
     (which acts on each input channel separately).
     The `depth_multiplier` argument controls how many
     output channels are generated per input channel in the depthwise step.