[libadwaita] Specify the libawaita building branch

[Alexander Mikhaylenko <alexm src gnome org>]

commit fce1de5138bdee04097ace1d004a1383236ba3d8
Author: Tiago Gomes <tiago gomes codethink co uk>
Date:   Wed Jun 23 11:30:29 2021 +0100

    Specify the libawaita building branch
    
    Otherwise it defaults to the "master" branch which does not exist.

 examples/org.gnome.Adwaita.Demo.json | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/examples/org.gnome.Adwaita.Demo.json b/examples/org.gnome.Adwaita.Demo.json
index 9b86dd68..5e2c035a 100644
--- a/examples/org.gnome.Adwaita.Demo.json
+++ b/examples/org.gnome.Adwaita.Demo.json
@@ -41,7 +41,8 @@
       "sources": [
         {
           "type": "git",
-          "url": "https://gitlab.gnome.org/GNOME/libadwaita.git";
+          "url": "https://gitlab.gnome.org/GNOME/libadwaita.git";,
+          "branch": "main"
         }
       ]
     }