[gnome-control-center] flatpak: Fix NetworkManager branch name

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

commit 0f6394d609c4c00f4a55cd9f02e8af8328a1ef40
Author: 1pav <40791-1pav users noreply gitlab gnome org>
Date:   Thu May 20 18:45:49 2021 +0200

    flatpak: Fix NetworkManager branch name
    
    Upstream branch of NetworkManager module has been renamed
    from 'master' to 'main'. This caused NetworkManager build
    to fail.

 build-aux/flatpak/org.gnome.Settings.json | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/build-aux/flatpak/org.gnome.Settings.json b/build-aux/flatpak/org.gnome.Settings.json
index fae5e16a7..e64bd9fda 100644
--- a/build-aux/flatpak/org.gnome.Settings.json
+++ b/build-aux/flatpak/org.gnome.Settings.json
@@ -376,6 +376,7 @@
             "sources" : [
                 {
                     "type" : "git",
+                    "branch": "main",
                     "url" : "https://gitlab.freedesktop.org/NetworkManager/NetworkManager.git";
                 }
             ]