Bug#586172: add example of -timestamp format to man page

[jidanni at jidanni.org]

Package: ffmpeg
Version: 5:0.6~svn20100603-0.0
Severity: wishlist
File: /usr/share/man/man1/ffmpeg.1.gz

We see
       -timestamp time
           Set the timestamp.

but without adding an example, on cannot guess just what timestamp
format we should use.