docs/sp_SP: Remove ZERO WIDTH SPACE in memory-barriers.txt

docs/sp_SP: Remove ZERO WIDTH SPACE in memory-barriers.txt

As this file is included literally, ZERO WIDTH SPACE causes
"make pdfdocs" to emit messages which read:

  Missing character: There is no ​ (U+200B) in font DejaVu Sans Mono/OT:script=latn;language=dflt;!
  Missing character: There is no ​ (U+200B) in font DejaVu Sans Mono/OT:script=latn;language=dflt;!

U+200B (ZERO WIDTH SPADE) has no effect in literal blocks.
Remove them and get rid of those noises.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Acked-by: Carlos Bilbao <carlos.bilbao@amd.com>
Link: https://lore.kernel.org/r/c38176c7-c30a-4c2c-3516-8d3be1c267dc@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>