fix: update help msg description about pullCmd() (#438)

Signed-off-by: Juncheng Zhu <[email protected]>

cmd/oras/pull.go

@@ -50,15 +50,9 @@ func pullCmd() *cobra.Command {
 		Short: "Pull files from remote registry",
 		Long: `Pull files from remote registry
 
-Example - Pull only files with the "application/vnd.oci.image.layer.v1.tar" media type (default):
+Example - Pull all files:
   oras pull localhost:5000/hello:latest
 
-Example - Pull only files with the custom "application/vnd.me.hi" media type:
-  oras pull localhost:5000/hello:latest -t application/vnd.me.hi
-
-Example - Pull all files, any media type:
-  oras pull localhost:5000/hello:latest -a
-
 Example - Pull files from the insecure registry:
   oras pull localhost:5000/hello:latest --insecure