Prefer long flags in quick start example (#1029)

`--detach` is clearer (to a new Docker user) than `-d`. Similarly `--volume` is clearer than `-v`.

Motivation: https://changelog.com/posts/use-long-flags-when-scripting
Referred to: https://docs.docker.com/engine/reference/commandline/run/

README.md

@@ -25,9 +25,9 @@ With watchtower you can update the running version of your containerized app sim
 Watchtower will pull down your new image, gracefully shut down your existing container and restart it with the same options that were used when it was deployed initially. Run the watchtower container with the following command:
 
 ```
-$ docker run -d \
+$ docker run --detach \
     --name watchtower \
-    -v /var/run/docker.sock:/var/run/docker.sock \
+    --volume /var/run/docker.sock:/var/run/docker.sock \
     containrrr/watchtower
 ```