small fix.

python/paddle/trainer_config_helpers/layers.py

@@ -2341,7 +2341,7 @@ def cos_sim(a, b, scale=1, size=1, name=None, layer_attr=None):
 def l2_distance_layer(x, y, name=None, layer_attr=None):
     """
     This layer calculates and returns the Euclidean distance between two input
-    vectors a and b. The equation is as follows:
+    vectors x and y. The equation is as follows:
 
     ..  math::
         l2_distance(\\mathbf{x}, \\mathbf{y}) = \\sqrt{\\sum_{i=1}^D(x_i - y_i)}
@@ -2372,7 +2372,7 @@ def l2_distance_layer(x, y, name=None, layer_attr=None):
     :rtype: LayerOutput
     """
 
-    assert isinstance(x, LayerOutput) and isinstance(x, LayerOutput)
+    assert isinstance(x, LayerOutput) and isinstance(y, LayerOutput)
     Layer(
         name=name,
         type=LayerType.L2_DISTANCE,