docs: xref doesn't render in title

The `post-build-hook` text currently appears in the index, but not on the actual title line of the section, this follows the pattern used in a previous section to get a reference into a title.

doc/manual/advanced-topics/post-build-hook.xml

@@ -5,7 +5,7 @@
       version="5.0"
       >
 
-<title>Using the <xref linkend="conf-post-build-hook" /></title>
+<title>Using the <option linkend="conf-post-build-hook">post-build-hook</option></title>
 <subtitle>Uploading to an S3-compatible binary cache after each build</subtitle>