Documentation: lockstat: Fix typo lokcing -> locking

Cc: Jiri Kosina <trivial@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>