[gtk-doc] cmake: fix typo when using gtkdoc-fixxref

[Corentin Noël <corentinnoel src gnome org>]

commit 91c18b9b4df14d0cafaddb2c6d173aa231f58bc4
Author: Corentin Noël <corentin elementary io>
Date:   Fri Oct 5 15:14:49 2018 +0200

    cmake: fix typo when using gtkdoc-fixxref

 buildsystems/cmake/GtkDocConfig.cmake.in | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/buildsystems/cmake/GtkDocConfig.cmake.in b/buildsystems/cmake/GtkDocConfig.cmake.in
index 9d41696..0df276d 100644
--- a/buildsystems/cmake/GtkDocConfig.cmake.in
+++ b/buildsystems/cmake/GtkDocConfig.cmake.in
@@ -290,7 +290,7 @@ function(gtk_doc_add_module _doc_prefix)
             cd ${_output_dir} && ${GTKDOC_FIXXREF_EXE}
                 --module=${_doc_prefix}
                 --module-dir=${_output_html_dir}
-                ${_fixxref_opts}
+                ${_fixxrefopts}
         COMMENT
             "Generating HTML documentation for ${_doc_prefix} module with gtkdoc-mkhtml"
         VERBATIM)