docs: Add colon clearing sphinx warning

Sphinx emits various warnings all caused by a missing colon before code
block:

	WARNING: Block quote ends without a blank line; unexpected unindent.
	ERROR: Unexpected indentation.
	WARNING: Block quote ends without a blank line; unexpected unindent.

Add the colon, clearing sphinx warnings.
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

include/linux/wait.h

@@ -101,7 +101,7 @@ init_waitqueue_func_entry(struct wait_queue_entry *wq_entry, wait_queue_func_t f
  * lead to sporadic and non-obvious failure.
  *
  * Use either while holding wait_queue_head::lock or when used for wakeups
- * with an extra smp_mb() like:
+ * with an extra smp_mb() like::
  *
  *      CPU0 - waker                    CPU1 - waiter
  *