Documentation: accel: escape wildcard in special file path

Stephen Rothwell reported htmldocs warning then merging accel tree:

Documentation/accel/introduction.rst:72: WARNING: Inline emphasis start-string without end-string.

Sphinx confuses the file wildcards with inline emphasis (italics), hence
the warning.

Fix the warning by escaping wildcards.

Link: https://lore.kernel.org/linux-next/20230120132116.21de1104@canb.auug.org.au/


Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>