Adjust broken link in service.md

Author: victuos

content/en/docs/concepts/services-networking/service.md

@@ -428,7 +428,7 @@ variables and DNS.
 
 When a Pod is run on a Node, the kubelet adds a set of environment variables
 for each active Service.  It supports both [Docker links
-compatible](https://docs.docker.com/userguide/dockerlinks/) variables (see [makeLinkVariables](https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/envvars/envvars.go#L72))
+compatible](https://docs.docker.com/userguide/dockerlinks/) variables (see [makeLinkVariables](https://github.com/kubernetes/kubernetes/blob/dd2d12f6dc0e654c15d5db57a5f9f6ba61192726/pkg/kubelet/envvars/envvars.go#L72))
 and simpler `{SVCNAME}_SERVICE_HOST` and `{SVCNAME}_SERVICE_PORT` variables,
 where the Service name is upper-cased and dashes are converted to underscores.