libtracefs Documentation: Fix tracefs_cpu_snapshot_open() man pages

The tracefs_cpu_snapshot_open() prototype in the man page had a typo and
it was also missing from the main libtracefs man page.

Link: https://lore.kernel.org/linux-trace-devel/20240108230652.51bc3b6a@gandalf.local.home

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>