pre-built mutation annotated tree README doesn't link to correct references

[jbloom]

The pre-built mutation annotated trees at http://hgdownload.soe.ucsc.edu/goldenPath/wuhCor1/UShER_SARS-CoV-2/ has a README that says to cite Lanfear (2020) and Turakhia et al (2021).

But it seems like McBroome et al (2021) is also a relevant reference for these pre-built trees (maybe the most relevant one?) and should be listed in README too?

[AngieHinrichs]

Yes, thank you for pointing that out @jbloom! I will update it.