Fix broken link of tf.train.Example in recurrent_quickdraw.md (#17678)

In recurrent_quickdraw.md, the link `${tf.train.Example}` is
not rendered correctly. This fix fixes the broken link with
correct rendering. Now the link is rendered the same way
as tf.train.Example in api_guides/python/reading_data.md and extend/new_data_formats.md

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