tls13: fix a wrong RFC reference section

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 3c2696f..0ca802b 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -447,7 +447,7 @@
 
 /*
  * TLS 1.3 signature algorithms
- * RFC 8446, Section 4.2.2
+ * RFC 8446, Section 4.2.3
  */
 
 /* RSASSA-PKCS1-v1_5 algorithms */