MAINT: Rename default branch

[jklenzing]

Description

Updates documentation after renaming default branch to main

Type of change

• This change is a documentation update

How Has This Been Tested?

via travis

Checklist:

• Make sure you are merging into the develop (not master) branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• Add a note to CHANGELOG.md, summarizing the changes

[jklenzing]

Should be ready to go

[rstoneback]

Thanks!