Fix build of purism example
Specifying custom branch name main
in example manifest to fix flatpak-builder
error
Downloading sources
Fetching full git repo https://gitlab.gnome.org/GNOME/libhandy.git
fatal: ambiguous argument 'master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Failed to download sources: module libhandy: Child process exited with code 128