[gnome-tour] cargo: revert the change to 2021 edition

[Bilal Elmoussaoui <bilelmoussaoui src gnome org>]

commit be014fca63e168cec64eeb66de6d5383097fb2f9
Author: Bilal Elmoussaoui <bil elmoussaoui gmail com>
Date:   Thu Jan 27 09:33:48 2022 +0000

    cargo: revert the change to 2021 edition
    
    See https://gitlab.gnome.org/GNOME/gnome-build-meta/-/merge_requests/1498#note_1367023

 Cargo.toml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/Cargo.toml b/Cargo.toml
index b81201b..55109ab 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,7 +2,7 @@
 name = "gnome-tour"
 version = "40.0.0"
 authors = ["Bilal Elmoussaoui <bil elmoussaoui gmail com>"]
-edition = "2021"
+edition = "2018"
 
 
 [dependencies]