Update Data.md

Author: macaba

docs/protocols/Data.md

@@ -47,6 +47,6 @@ u8[] message;
 ```
 version: 1
 level:  0 trace, 1 debug, 2 information, 3 warning, 4 error, 5 critical
-message: ASCII, not null-terminated (header length value determines length of string)
+message: ASCII, not null-terminated (header payload length value determines length of string)
 ```