read owner from the organization input parameter

[maxdanilov]

A follow up for #25 and #27.

github_token was not the only input parameter to fix. Another one is about owner.

In order to keep compatibility with the behaviour with the previous shell version of the action, action.yml and README.md, reading the owner name from the organization input instead of owner.

[benbalter]

Thanks once again @maxdanilov. Eventually, to support #7, it may be best to document owner but fall back to organization if set to maintain backward compatibility.