[gnome-builder] build: use "main" for ostree branch

[Christian Hergert <chergert src gnome org>]

commit 5f16ed8cb9e6877dfdbc6f9d7caa626f310d9dfc
Author: Christian Hergert <chergert redhat com>
Date:   Wed May 12 12:44:21 2021 -0700

    build: use "main" for ostree branch

 build-aux/flatpak/org.gnome.Builder.json | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/build-aux/flatpak/org.gnome.Builder.json b/build-aux/flatpak/org.gnome.Builder.json
index 24f59c0f3..f1986476d 100644
--- a/build-aux/flatpak/org.gnome.Builder.json
+++ b/build-aux/flatpak/org.gnome.Builder.json
@@ -470,7 +470,8 @@
             "sources" : [
                 {
                     "type" : "git",
-                    "url" : "https://github.com/ostreedev/ostree.git";
+                    "url" : "https://github.com/ostreedev/ostree.git";,
+                    "branch" : "main"
                 }
             ]
         },