message: fix a typo in tf.nn.separable_conv2d's doc
  It's obvious that the right bracket in "sum_{di, dj, q, r]" should
  be a right brace.
diff: @@ -417,7 +417,7 @@ def separable_conv2d(input,
  In detail,
  - output[b, i, j, k] = sum_{di, dj, q, r]
  + output[b, i, j, k] = sum_{di, dj, q, r}
  input[b, strides[1] * i + di, strides[2] * j + dj, q] *
  depthwise_filter[di, dj, q, r] *
  pointwise_filter[0, 0, q * channel_multiplier + r, k]
  