poke: change the short form of --freshness to -f

For consistency with ndnpingserver and ndnputchunks

Change-Id: I82afc6de29e207d65bf885b0acbabfc9da05a5a7
diff --git a/manpages/ndnpoke.rst b/manpages/ndnpoke.rst
index b5c105d..39af974 100644
--- a/manpages/ndnpoke.rst
+++ b/manpages/ndnpoke.rst
@@ -4,7 +4,7 @@
 Synopsis
 --------
 
-**ndnpoke** [-h] [-x *freshness*] [-F] [-S *info*] [-u\|\ -w *timeout*] [-v] [-V] *name*
+**ndnpoke** [-h] [-f *freshness*] [-F] [-S *info*] [-u\|\ -w *timeout*] [-v] [-V] *name*
 
 Description
 -----------
@@ -20,7 +20,7 @@
 ``-h, --help``
   Print help and exit.
 
-``-x, --freshness <freshness>``
+``-f, --freshness <freshness>``
   Set ``freshness`` (in milliseconds) as the ``FreshnessPeriod``.
 
 ``-F, --final``