tools/memory-model: Fix smp_mb__after_spinlock() spelling

A misspelled git-grep regex revealed that smp_mb__after_spinlock()
was misspelled in explanation.txt.  This commit adds the missing "_".

Fixes: 1c27b64 ("Automate memory-barriers.txt; provide Linux-kernel memory model")
[ paulmck: Apply Alan Stern commit-log feedback. ]
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/memory-model/Documentation/explanation.txt

@@ -2510,7 +2510,7 @@ they behave as follows:
 	smp_mb__after_atomic() orders po-earlier atomic updates and
 	the events preceding them against all po-later events;
 
-	smp_mb_after_spinlock() orders po-earlier lock acquisition
+	smp_mb__after_spinlock() orders po-earlier lock acquisition
 	events and the events preceding them against all po-later
 	events.