Update cmd/README w/updated example URLs

Updated example URLs to latest locations

cmd/README.md

@@ -10,9 +10,9 @@ It's common to have a small `main` function that imports and invokes the code fr
 
 Examples:
 
-* https://github.com/heptio/ark/tree/master/cmd (just a really small `main` function with everything else in packages)
+* https://github.com/vmware-tanzu/velero/tree/main/cmd (just a really small `main` function with everything else in packages)
 * https://github.com/moby/moby/tree/master/cmd
-* https://github.com/prometheus/prometheus/tree/master/cmd
+* https://github.com/prometheus/prometheus/tree/main/cmd
 * https://github.com/influxdata/influxdb/tree/master/cmd
 * https://github.com/kubernetes/kubernetes/tree/master/cmd
 * https://github.com/dapr/dapr/tree/master/cmd