doc: fix build with sphinx 4.5

doc: fix build with sphinx 4.5

Latest Sphinx checks C language syntax more aggressively.
Fix the following warning by correcting C language syntax.

doc/guides/prog_guide/event_ethernet_rx_adapter.rst:243:
WARNING: Could not lex literal_block as "c". Highlighting skipped.

Fixes: 3c838062b91f ("eventdev: introduce event vector Rx capability")
Cc: stable@dpdk.org
Signed-off-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Jay Jayatheerthan <jay.jayatheerthan@intel.com>