Change github MASTER links to MAIN in the handbook

[Stefan Pejcic]

https://github.com/github/renaming

Master is replaced with main, so all links should be changed, for example:

• current:
  https://github.com/wp-cli/handbook/edit/master/hosting-companies.md

• should be:
  https://github.com/wp-cli/handbook/edit/main/hosting-companies.md

[Daniel Bachhuber]

Thanks for the report, @stefanpejcic!

Also tracking this with https://meta.trac.wordpress.org/ticket/6394

[Daniel Bachhuber]

These links are now fixed 🐛 🔨