cgroups: fix CONTENTS in cgroups documentation

Add a forgotten item into CONTENTS.
Signed-off-by: NKirill A. Shutemov <kirill@shutemov.name>
Acked-by: NPaul Menage <menage@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroups/cgroups.txt

@@ -22,6 +22,7 @@ CONTENTS:
 2. Usage Examples and Syntax
   2.1 Basic Usage
   2.2 Attaching processes
+  2.3 Mounting hierarchies by name
 3. Kernel API
   3.1 Overview
   3.2 Synchronization