docs/rcu.txt: Fix minor typo

s/presented/prevented/
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Message-Id: <20161018050418.4912-1-bobby.prani@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

docs/rcu.txt

@@ -145,7 +145,7 @@ The core RCU API is small:
         and then read from there.
 
         RCU read-side critical sections must use atomic_rcu_read() to
-        read data, unless concurrent writes are presented by another
+        read data, unless concurrent writes are prevented by another
         synchronization mechanism.
 
         Furthermore, RCU read-side critical sections should traverse the