post-receive-email example hook: sed command for getting description was wrong

post-receive-email example hook: sed command for getting description was wrong

The sed command that extracted the first line of the project description
didn't include the -n switch and hence the project name was being
printed twice.  This was ruining the email header generation because it
was assumed that the description was only one line and was included in
the subject.  This turned the subject into a two line item and
prematurely finished the header.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>