doc: Remove one use of 'file-append*'.

* doc/build.scm (texinfo-manual-source)[build]: Use 'file-append', not
'file-append*', for 'htmlxref.cnf', to ensure it has the right basename.

doc/build.scm

@@ -141,7 +141,7 @@ as well as images, OS examples, and translations."
                             (date->string date "~B ~Y")
                             version version))))))
 
-          (install-file #$(file-append* documentation "/htmlxref.cnf")
+          (install-file #$(file-append documentation "/htmlxref.cnf")
                         #$output)
 
           (for-each (lambda (texi)