Remove link to nonexistent rocket driver docs

The rocket driver and documentation were removed in this commit, but
the corresponding entry in index.rst was not removed.

Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Fixes: 3b00b6af

 ("tty: rocket, remove the driver")
Link: https://lore.kernel.org/r/20210511134937.2442291-1-desmondcheongzx@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>