khan: fix comment
It's a little atom; so 31 bits, not 32.
** [%move move-args=*]
** == ==
**
-** request-id is a 32-bit client-supplied identifier that will
+** request-id is a 31-bit client-supplied identifier that will
** be returned along with the response, to allow correlating
** responses to simultaneous requests. (any request that may
** take more than a single arvo event is not guaranteed to
