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/[e-mail]/
Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Bagas Sanjaya <[e-mail]>
Reviewed-by: Jeffrey Hugo <[e-mail]>
Signed-off-by: Oded Gabbay <[e-mail]>

Documentation/accel/introduction.rst

···
 The accelerator devices will be exposed to the user space with the dedicated
 261 major number and will have the following convention:
 
-- device char files - /dev/accel/accel*
-- sysfs             - /sys/class/accel/accel*/
-- debugfs           - /sys/kernel/debug/accel/*/
+- device char files - /dev/accel/accel\*
+- sysfs             - /sys/class/accel/accel\*/
+- debugfs           - /sys/kernel/debug/accel/\*/
 
 Getting Started
 ===============