Update manpage date.

We will have another release soon. So, let's care for details better to
soon than too late.

docs/manpage.rst

@@ -3,7 +3,7 @@
 :subtitle: create passphrases
 :manual_section: 1
 :manual_group: User Commands
-:date: August 2017
+:date: September 2017
 :version: diceware 0.9.2.dev0
 :author: Written by Uli Fouquet and contributors