[PATCH memory-model] Fix smp_mb__after_spinlock() spelling

[Paul E. McKenney]

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

Fixes: 1c27b644c0fd ("Automate memory-barriers.txt; provide Linux-kernel memory model")
[ paulmck: Apply Alan Stern commit-log feedback. ]
Signed-off-by: Björn Töpel <bjorn.topel@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
index f9d610d5a1a4..5d72f3112e56 100644
--- a/tools/memory-model/Documentation/explanation.txt
+++ b/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.