[Bugfix] Update match for enterprise GH URLs

[hollabaq86]

👋 When trying to use the gem for a public GitHub repo, I encountered the following error:

Traceback (most recent call last):
	9: from /usr/local/bin/master_to_main:23:in `<main>'
	8: from /usr/local/bin/master_to_main:23:in `load'
	7: from /Library/Ruby/Gems/2.6.0/gems/master_to_main-0.0.7/exe/master_to_main:5:in `<top (required)>'
	6: from /Library/Ruby/Gems/2.6.0/gems/thor-1.0.1/lib/thor/base.rb:485:in `start'
	5: from /Library/Ruby/Gems/2.6.0/gems/thor-1.0.1/lib/thor.rb:392:in `dispatch'
	4: from /Library/Ruby/Gems/2.6.0/gems/thor-1.0.1/lib/thor/invocation.rb:127:in `invoke_command'
	3: from /Library/Ruby/Gems/2.6.0/gems/thor-1.0.1/lib/thor/command.rb:27:in `run'
	2: from /Library/Ruby/Gems/2.6.0/gems/master_to_main-0.0.7/lib/master_to_main/cli.rb:14:in `update'
	1: from /Library/Ruby/Gems/2.6.0/gems/master_to_main-0.0.7/lib/master_to_main/cli.rb:89:in `prompt_info'
/Library/Ruby/Gems/2.6.0/gems/master_to_main-0.0.7/lib/master_to_main/cli.rb:72:in `get_github_info': undefined method `split' for nil:NilClass (NoMethodError)

On investigation, current behavior is expecting a : in the URL of the repo being updated, which isn't the case for public-facing repos (i.e. github.com/dewyze/master_to_main).

This PR:

• fixes up this issue with a match statement (I though a specific check for com: would be enough to differentiate between GHE URLs and public domains)
• Clarifies the documentation a bit (I got a little confused updating repos that are owned by an organization and not me)
• Updates the CHANGELOG and CONTRIBUTORs files.

Thanks for making this gem!