Change master to main in installation guide

[RhysMacMillan]

When I was building cyclus from source using this link, the dated default branch "master" has not been updated to "main" per the new standard. This inconsistency occurs in the first step of installing both cyclus and cycamore on that page.

[gonuke]

We probably need to review all the code on the website: https://github.com/search?q=repo%3Acyclus%2Fcyclus.github.com%20master&type=code

[gonuke]

Looks like we missed a few in #341