DocBook: Change a headline in kernel-locking to list all three main types of locking.

Change a headline to reflect that there are three main types of kernel
locking, not two.
Signed-off-by: NLinus Nilsson <lajnold@gmail.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/DocBook/kernel-locking.tmpl

@@ -219,7 +219,7 @@
    </para>
 
    <sect1 id="lock-intro">
-   <title>Two Main Types of Kernel Locks: Spinlocks and Semaphores</title>
+   <title>Three Main Types of Kernel Locks: Spinlocks, Mutexes and Semaphores</title>
 
    <para>
      There are three main types of kernel locks.  The fundamental type