Change github MASTER links to MAIN in the handbook

[stefanpejcic]

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

[danielbachhuber]

Thanks for the report, @stefanpejcic!

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

[danielbachhuber]

These links are now fixed 🐛 🔨