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: 1c27b644 ("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>