Fix doc format issue in enable_eager_execution (#19116)

* Fix doc format issue in enable_eager_execution

This fix tries to address the issue raised in 19022 where
the doc of `tf.enable_eager_execution` was not well formatted.
It seems the additional empty lines may confuse the doc generator.
This fix fixes with proper alignment and removal of empty lines.

This fix fixes 19022.

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

* Remove extra spaces

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