input: docs: pxrc: remove reference to phoenix-sim

The reference undeniably points to something unrelated nowadays.
Remove it.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Suggested-by: Mark Olsson <mark@markolsson.se>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230824-pxrc-doc-v1-1-038b75a2ef05@gmail.com