Doc: Clusters from Scratch: Update index.rst.

Fixes T910

Author: clumens

doc/sphinx/Clusters_from_Scratch/index.rst

@@ -11,7 +11,7 @@ The example cluster will use:
 
 * |CFS_DISTRO| |CFS_DISTRO_VER| as the host operating system
 * Corosync to provide messaging and membership services
-* Pacemaker 2 as the cluster resource manager
+* Pacemaker 3 as the cluster resource manager
 * DRBD as a cost-effective alternative to shared storage
 * GFS2 as the cluster filesystem (in active/active mode)