doc: Fix typo in rcutorture documentation

Signed-off-by: Lihao Liang <lianglihao@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

tools/testing/selftests/rcutorture/doc/rcu-test-image.txt

-This document describes one way to created the rcu-test-image file
+This document describes one way to create the rcu-test-image file
 that contains the filesystem used by the guest-OS kernel.  There are
 probably much better ways of doing this, and this filesystem could no
 doubt be smaller.  It is probably also possible to simply download