Fix error for env variables example in docker reference
@@ -361,7 +361,7 @@ RUN /bin/bash -c 'source $HOME/.bashrc ; echo $HOME'
> This means that normal shell processing does not happen. For example,
> `RUN [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`.
> If you want shell processing then either use the *shell* form or execute
-> a shell directly, for example: `RUN [ "sh", "-c", "echo", "$HOME" ]`.
+> a shell directly, for example: `RUN [ "sh", "-c", "echo $HOME" ]`.
The cache for `RUN` instructions isn't invalidated automatically during
the next build. The cache for an instruction like
