Fix ulimit command form
The ulimit is builtin, so we need shell form to execute it.
@@ -539,7 +539,7 @@ available in the default container, you can set these using the `--ulimit` flag.
`--ulimit` is specified with a soft and hard limit as such:
`<type>=<soft limit>[:<hard limit>]`, for example:
- $ docker run --ulimit nofile=1024:1024 --rm debian ulimit -n
+ $ docker run --ulimit nofile=1024:1024 --rm debian sh -c "ulimit -n"
1024
> **Note:**
