[gnome-todo] build: Don't try to use 'gtk_doc' anymore

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit c8113553b8eae8a0d21e442462da11dff024b374
Author: Georges Basile Stavracas Neto <georges stavracas gmail com>
Date:   Wed Jun 20 23:46:54 2018 -0300

    build: Don't try to use 'gtk_doc' anymore
    
    This should've gone together with the gtk_doc → documentation
    renaming commit.

 meson.build | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/meson.build b/meson.build
index 475975e..0e0ddfa 100644
--- a/meson.build
+++ b/meson.build
@@ -164,8 +164,8 @@ subdir('src')
 subdir('data')
 subdir('po')
 
-enable_gtk_doc = get_option('gtk_doc')
-if enable_gtk_doc
+enable_documentation = get_option('documentation')
+if enable_documentation
   subdir('doc/reference')
 endif
 
@@ -182,7 +182,7 @@ output += '\n'
 output += '    Options:\n'
 output += '\n'
 output += '        Tracing.................. ' + (gnome_todo_tracing).to_string() + '\n'
-output += '        Documentation............ ' + enable_gtk_doc.to_string() + '\n'
+output += '        Documentation............ ' + enable_documentation.to_string() + '\n'
 output += '\n'
 output += '    Plugins:\n'
 output += '\n'