pkt-line: replace "stateless separator" with "response end"

In 0181b600a6 (pkt-line: define PACKET_READ_RESPONSE_END, 2020-05-19),
the Response End packet was defined for Git's network protocol. When the
patch was sent, it included an oversight where the error messages
referenced "stateless separator", the work-in-progress name, over
"response end", the final name chosen.

Correct these error messages by having them correctly reference
a "response end" packet.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

serve.c

@@ -218,7 +218,7 @@ static int process_request(void)
 			state = PROCESS_REQUEST_DONE;
 			break;
 		case PACKET_READ_RESPONSE_END:
-			BUG("unexpected stateless separator packet");
+			BUG("unexpected response end packet");
 		}
 	}