doc: fix example name

Commit cde5863eec77cab48fb846e4200ab6f0e5f3dbcf renamed the loop.ptt test
into loop-tnt.ptt and forgot to update the documentation that uses this test
as an example.  Update the documentation.

This fixes issue #3 reported by Baris Kasikci.

Change-Id: I2cad7d697b67a74a8d71f7c1ddcc47b69263b538
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>