Remove --rm from docker run --rm busybox echo hello world

This commit reverts (ca7997e).

The --rm tells docker to remove the container when the container
exits. Without --rm docker keeps around the container, and a user
can restart the container. In this chapter the reader does not
restart containers, so --rm keeps tidy the storage docker keeps
by deleting the stopped containers.

However, adding `--rm` increases how much the user new to docker
has to comprehend in the `docker run` commands. And keeping small
the number of new things a reader must comprehend makes it more
manageable for the reader to follow along.

Author: zipperer

chapter_09_docker.asciidoc

@@ -496,7 +496,7 @@ Follow those, and then test your installation by running:
 
 [subs="specialcharacters,macros"]
 ----
-$ pass:quotes[*docker run --rm busybox echo hello world*]
+$ pass:quotes[*docker run busybox echo hello world*]
 Unable to find image 'busybox:latest' locally
 [...]
 latest: Pulling from library/busybox