Improve --help output to include an example with Organisation

[Grundlefleck]

Hi,

I used your script successfully to import all the issues at https://github.com/MutabilityDetector/MutabilityDetector/issues , very happy, thank you.

To get it to work, I had to dig into the source of the script to discover that repositories belonging to an organisation, rather than a user, have a special syntax.

Normal user has syntax:
./migrateissues.py google_project github_user github_project

Whereas organisations need:
./migrateissues.py google_project github_user github_organisation/github_project

It would be nice if this was documented in the output of "migrateissues.py --help" so as not to require studying the source code.

Many thanks for a great utility.