readme: update doc link to pkg.go.dev
The older godoc.org encourages the user to use pkg.go.dev instead.
Change our link to point directly there (and use https).
@@ -33,7 +33,7 @@ go test -tags luminous ....
## Documentation
Detailed documentation is available at
-<http://godoc.org/github.com/ceph/go-ceph>.
+<https://pkg.go.dev/github.com/ceph/go-ceph>.
### Connecting to a cluster
