Fix copy/pasto in API help and a typo in message name

Fix copy/pasto in API help and a typo in message name

* The help for [[Special:ApiHelp/xml]] used the same copypasted
apihelp-json-param-callback message for both parameters instead of the
correct ones for those parameters.
* The message name in en.json and qqq.json was truncated, so that it did
not correspond to the parameter name. I presume this was unintentional
and not some strange limitation.

Change-Id: I666b2fd6511f08bccc9f05739bab68d7675ef5c6