Fix API validation response for team join message

[fitztrev]

If a team has "Manually review admission requests" enabled, the message field is required in the POST team join request.

http -A bearer -a lip_bobby post \
  http://localhost:8080/team/team-that-requires-a-message/join \
  message="too short"

{
    "error": {
        "message": [
            "Must be at least ArraySeq(30) characters long"
        ]
    }
}

-  Must be at least ArraySeq(30) characters long
+  Must be at least 30 characters long

[kraktus]

Not sure if this is an easy issue, the constraint is defined here https://github.com/lichess-org/lila/blob/5ed87699dad51ccf06e103f712fc304c009fed51/modules/team/src/main/TeamForm.scala#L33

and I see nothing obvious explaining the mis-formatting