tools/memory-model/README: Fix typo

Fix a trivial typo.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Author: akiyks

tools/memory-model/README

@@ -79,7 +79,7 @@ Several thousand more example litmus tests are available here:
 	https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/herd
 	https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/litmus
 
-Documentation describing litmus tests and now to use them may be found
+Documentation describing litmus tests and how to use them may be found
 here:
 
 	tools/memory-model/Documentation/litmus-tests.txt