Fix incorrect doc in tf.layers.dense

This fix tries to address the issue raised in 21525 where
the doc in tf.layers.dense is not correct. Specifically
`outputs = activation(inputs.kernel + bias) Where` ->
`outputs = activation(inputs * kernel + bias) where`

This fix fixes 21525.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>