Fix incorrect UUID index entry in function documentation

Hi,

Both the UUID data type and UUID functions pages define an index entry
for "UUID" that points to the data type section. As a result, the index
includes two identical entries linking to the UUID type docs,
which seems strange.

I believe the UUID functions page should instead define its own index
entry that links to itself. Currently, the indexterm is written as:

--------------------
  <sect1 id="functions-uuid">
   <title>UUID Functions</title>

   <indexterm zone="datatype-uuid">
    <primary>UUID</primary>
    <secondary>generating</secondary>
   </indexterm>
--------------------

I suspect that "datatype-uuid" is a copy-paste error and should be
"functions-uuid" to reflect the correct section. The attached patch
updates this accordingly.

Thoughts?

Regards,

-- 
Fujii Masao
NTT DATA Japan Corporation