Fix avro converter typo

[serprex]

peerdb/flow/model/qvalue/avro_converter.go

Lines 77 to 83 in 6dbdf61

	case QValueKindHStore, QValueKindJSON, QValueKindStruct:
	return &QValueKindAvroSchema{
	AvroLogicalSchema: map[string]interface{}{
	"type": "string",
	"values": "string",
	},
	}, nil

https://avro.apache.org/docs/1.11.0/spec.html#Maps

If hstore/json/struct should be encoded as string, AvroLogicalSchema should only be "string"

As is, it looks like the intent of the code is to be a map, but then "type" should be "map"

[serprex]

If type being string is correct, #846 fixes this