Freedesktop SDK example in documentation is broken on master

Summary

See https://docs.buildstream.build/master/examples/flatpak-autotools.html

Many outputs give:

Failed to load Source plugin 'ostree': The 'bst-plugins-experimental' distribution was not found and is required by the application

Steps to reproduce

What is the current bug behavior?

What is the expected correct behavior?

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

---