rpc/listaddressgroupings: redefine inner-most array as ARR_FIXED

ARR_FIXED is for cases like this, where the elements are in an array for convenience, rather than due to being dynamically sized lists.

src/wallet/rpcwallet.cpp

@@ -540,7 +540,7 @@ static RPCHelpMan listaddressgroupings()
                     {
                         {RPCResult::Type::ARR, "", "",
                         {
-                            {RPCResult::Type::ARR, "", "",
+                            {RPCResult::Type::ARR_FIXED, "", "",
                             {
                                 {RPCResult::Type::STR, "address", "The bitcoin address"},
                                 {RPCResult::Type::STR_AMOUNT, "amount", "The amount in " + CURRENCY_UNIT},