Doc: Edit section title to not clash with an existing module name

A link to 'Qt 3D' may in some cases lead to a wrong page.

Change-Id: I7b05bcb7c31a63316c5936a165c72d83111f7783
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>
Reviewed-by: Martin Smith <martin.smith@qt.io>

doc/src/whatsnew/whatsnew56.qdoc

@@ -62,7 +62,7 @@
 
     \section1 New Features in Qt 5.6
 
-    \section2 Qt 3D
+    \section2 Qt 3D Module
     \list
      \li New buffer and attribute API.
      \li Support for \l{OpenGL Instanced Rendering}{OpenGL instanced rendering}