Fix error strings for SSL_R_TLS13_DOWNGRADE.

make_errors.go didn't seem to get run.

Change-Id: I12739fbab75b9f4898f73f206e404d101642b9c0
Reviewed-on: https://boringssl-review.googlesource.com/31184
Reviewed-by: Adam Langley <agl@google.com>

crypto/err/ssl.errordata

@@ -166,6 +166,7 @@ SSL,215,SSL_HANDSHAKE_FAILURE
 SSL,216,SSL_SESSION_ID_CONTEXT_TOO_LONG
 SSL,290,SSL_SESSION_ID_TOO_LONG
 SSL,276,TICKET_ENCRYPTION_FAILED
+SSL,297,TLS13_DOWNGRADE
 SSL,1049,TLSV1_ALERT_ACCESS_DENIED
 SSL,1050,TLSV1_ALERT_DECODE_ERROR
 SSL,1021,TLSV1_ALERT_DECRYPTION_FAILED