Update the examples about the mount EDF option. (#196)

I noticed that the current CE documentation is missing how to specify
multiple mount points. This PR adds this example.

Author: gwangmu

docs/software/container-engine/edf.md

@@ -142,14 +142,19 @@ List of mounts in the format `SOURCE:DESTINATION[:FLAGS]`. By default, it perfor
 
      * Mounting the scratch filesystem using a host environment variable
         ```toml
-        mounts = ["${SCRATCH}:/scratch"]
+        mounts = ["${SCRATCH}:${SCRATCH}"]
         ```
 
      * Mounting a SquashFS image `${SCRATCH}/data.sqsh` to `/data`
         ```toml
         mounts = ["${SCRATCH}/data.sqsh:/data:sqsh"]
         ```
 
+     * Mounting multiple entities (the scratch filesystem and a SquashFS image)
+        ```toml
+        mounts = ["${SCRATCH}:${SCRATCH}", "${SCRATCH}/data.sqsh:/data:sqsh"]
+        ```
+
 !!! note
     * Mount flags are separated with a plus symbol, for example: `ro+private`.