docs: update link to RFC 4122 (#93)

Updates the README to fix the link to the RFC. Fixes #92.

README.md

@@ -1,6 +1,6 @@
 # uuid
 The uuid package generates and inspects UUIDs based on
-[RFC 4122](http://tools.ietf.org/html/rfc4122)
+[RFC 4122](https://datatracker.ietf.org/doc/html/rfc4122)
 and DCE 1.1: Authentication and Security Services. 
 
 This package is based on the github.com/pborman/uuid package (previously named