Small README fixes

[gpjacobs]

Hello and thank you for maintaining this helpful action!

I noticed a few small discrepancies in the documentation while using the action:

• The allowed values for links[].type are different in the v2.1 API: https://github.com/DataDog/schema/blob/main/service-catalog/v2.1/schema.json#L130
• The Opsgenie service_url property should be service-url
• The email contact examples should use type and contact (as documented in the table in this file)

Please let me know if you have any questions.

Thanks!