doc: specify default list output

Adding that the command given in the documentation lists the identifiers
of the available tapes.

Change-Id: Ia0ddffe48d72599ba81003102833696232fe72a1
Signed-off-by: Sebastien Gougeaud <sebastien.gougeaud@cea.fr>
Reviewed-on: https://cws-fleury.labs.ocre.cea.fr/gerrit/6572


Reviewed-by: Linter
Tested-by: Jenkins s8open_nr <s8open_nr@ccc.ocre.cea.fr>
Reviewed-by: Patrice LUCAS <patrice.lucas@cea.fr>
Reviewed-by: Thomas Leibovici <thomas.leibovici@cea.fr>

doc/README.md

@@ -173,7 +173,7 @@ cksum=md5:7c28aec5441644094064fcf651ab5e3e,user=foo
 ## Device and media management
 ### Listing resources
 Any device or media can be listed using the 'list' operation. For instance,
-the following will list all the existing tapes:
+the following will list all the existing tape identifiers:
 ```
 phobos tape list
 ```