fetch-pack.c: correct command at the beginning of an error message

One error message in fetch-pack.c uses 'git fetch_pack' at the beginning
which is not a git command.  Use 'git fetch-pack' instead.

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fetch-pack.c

@@ -240,7 +240,7 @@ static enum ack_type get_ack(int fd, unsigned char *result_sha1)
 			return ACK;
 		}
 	}
-	die(_("git fetch_pack: expected ACK/NAK, got '%s'"), line);
+	die(_("git fetch-pack: expected ACK/NAK, got '%s'"), line);
 }
 
 static void send_request(struct fetch_pack_args *args,