fix Docker CMD docs link (#124)

lib/index.js

@@ -95,7 +95,7 @@ export default class Now extends EventEmitter {
 
       if (!docker.some((cmd) => 'CMD' === cmd.name)) {
         const e = Error('No `CMD` found in `Dockerfile`. ' +
-          'See: https://docs.docker.com/engine/reference/builder/#/run');
+          'See: https://docs.docker.com/engine/reference/builder/#/cmd');
         e.userError = true;
         throw e;
       }