Update topology.py (keras-team#2373)

Author: EderSantana

keras/engine/topology.py

@@ -1169,7 +1169,7 @@ def _arguments_validation(self, layers, mode, concat_axis, dot_axes,
                     raise Exception('Invalid format for dot_axes - list elements should be "int".')
                 if shape1[dot_axes[0]] != shape2[dot_axes[1]]:
                     raise Exception('Dimension incompatibility using dot mode: ' +
-                                    '%s != %s. ' % (shape1[dot_axes[0]], shape2[dot_axes[1][i]]) +
+                                    '%s != %s. ' % (shape1[dot_axes[0]], shape2[dot_axes[1]]) +
                                     'Layer shapes: %s, %s' % (shape1, shape2))
         elif mode == 'concat':
             reduced_inputs_shapes = [list(shape) for shape in input_shapes]