Update docs link in readme

[brandond]

The Readme currently links to the old docs, which show v1.2.0 as the latest release. It should probably link to https://pkg.go.dev/github.com/robfig/cron/v3 instead? The go docs site is not nice enough to suggest the v3 module as a newer version of robfig/cron so some folks might not make the jump on their own.

[davidschlachter]

https://github.com/robfig/cron/pull/447