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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/serial/index.rst

@@ -19,7 +19,6 @@ Serial drivers
 
     moxa-smartio
     n_gsm
-    rocket
     serial-iso7816
     serial-rs485