doc: Tweak "cube" index entry

With the arrival of the CUBE key word/feature, the index entries for the
cube extension and the CUBE feature were collapsed into one.  Tweak the
entry for the cube extension so they are separate entries.

doc/src/sgml/cube.sgml

@@ -4,7 +4,7 @@
  <title>cube</title>
 
  <indexterm zone="cube">
-  <primary>cube</primary>
+  <primary>cube (extension)</primary>
  </indexterm>
 
  <para>