fix comment

Author: ulugbekna

src/not-so-smart/decoder.mli

@@ -3,8 +3,8 @@
 
     We define a "packet line" (aka a "packet") as
 
-      |   4 bytes   || (pkt-len)-4 |
-      [ enc-pkt-len ][ pkt-content ]
+      |   4 bytes   || (enc-pkt-len)-4 |
+      [ enc-pkt-len ][   pkt-content   ]
       |-------    pkt-len    ------|
 
     Example: "0009done\n" where [enc-pkt-len = 4] and [pkt-content = "done"] given we