git-send-email: Change the prompt for the subject of the initial message.

I never understood what this prompt was asking for until I read the actual
source code.  I think this wording is much more understandable.

Signed-off-by: Benoit Sigoure <tsuna@lrde.epita.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -303,7 +303,7 @@ sub expand_aliases {
 
 if (!defined $initial_subject && $compose) {
 	do {
-		$_ = $term->readline("What subject should the emails start with? ",
+		$_ = $term->readline("What subject should the initial email start with? ",
 			$initial_subject);
 	} while (!defined $_);
 	$initial_subject = $_;