[fractal] ci: Fix docs generation

[Kévin Commaille <kcommaille src gnome org>]

commit c9e0e5e775890f7a53fb82c7a32cce9907bd5bf6
Author: Kévin Commaille <zecakeh tedomum fr>
Date:   Thu May 12 11:21:04 2022 +0200

    ci: Fix docs generation

 src/meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/meson.build b/src/meson.build
index 6e1597c3e..03d0a5121 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -85,7 +85,7 @@ cargo_doc = custom_target(
     console: true,
     command: [
       'env', cargo_env + doc_env,
-      cargo, 'doc',
+      cargo, '+nightly', 'doc',
       cargo_options + doc_deps + ['--no-deps', '--document-private-items', '-Zrustdoc-map'] ,
     ],
   )