Make Dot documentation inline with Concatenate (#10271)

Doc expects a list containing 2 tensors.

keras/layers/merge.py

@@ -408,7 +408,7 @@ class Concatenate(_Merge):
 class Dot(_Merge):
     """Layer that computes a dot product between samples in two tensors.
 
-    E.g. if applied to two tensors `a` and `b` of shape `(batch_size, n)`,
+    E.g. if applied to a list of two tensors `a` and `b` of shape `(batch_size, n)`,
     the output will be a tensor of shape `(batch_size, 1)`
     where each entry `i` will be the dot product between
     `a[i]` and `b[i]`.