Fix nits in release notes
Removed the `*` in `tf.nest` which was not displayed as a bullet item in a sublist.

Removed spurious `:` at the end of some bullets to make it consistent throughout the relnotes.

Removed `tf.keras` in Keras section as it doesn't make sense to introduce one extra level of indentation for just one single bullet. This involves also removing 4 spaces in front of every bullet point in the corresponding section to make sure the content is not interpreted as code.

To be merged __only__ after RC0 is finalized

Signed-off-by: mihaimaruseac@google.com