bug#25872: [minor] SXML Transform section in ref-man does not mention `(sxml transform)'

[Matt Wette]

This is a minor nit.  The reference manual does not direct user to do `(use-modules (sxml transform))'

The following patch is wrt guile-2.1.7/doc/ref/sxml.texi

--- sxml.texi.orig 2017-02-25 06:33:38.000000000 -0800

+++ sxml.texi 2017-02-25 06:36:00.000000000 -0800

@@ -457,6 +457,8 @@

 @node Transforming SXML

 @subsection Transforming SXML

 @subsubsection Overview

address@hidden(sxml transform)} defines a number of procedures for transforming

+SXML, analogous to using XSL for XML.

 @heading SXML _expression_ tree transformers

 @subheading Pre-Post-order traversal of a tree and creation of a new tree

 @smallexample