Add Transient_background_example.ipynb to run_tutorials.yml

[israelmcmc]

Added in #460 but not in https://github.com/cositools/cosipy/blob/develop/docs/tutorials/run_tutorials.yml yet.

[Yong2Sheng]

@israelmcmc Thanks for taking care of it!

[israelmcmc]

No problem, thank you @greendw :)

[Yong2Sheng]

Thanks @greendw !