Doc: Updated index.html title to "Qt 5.6"

Change-Id: Iebe70377bf3c24ae9b6eaf99d1665942938bf178
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>

doc/src/index.qdoc

@@ -32,7 +32,7 @@
 \div {class="landing"}
     \div {class="col-1"}
             \div {class="heading"}
-                \title Qt 5.5
+                \title Qt 5.6
             \enddiv
 
                 Qt is a full development framework with tools designed to streamline