Fix wrong json key in receipt event

src/receipt.rs

@@ -22,6 +22,7 @@ pub type ReceiptEventContent = HashMap<EventId, Receipts>;
 #[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct Receipts {
     /// A collection of users who have sent *m.read* receipts for this event.
+    #[serde(rename="m.read")]
     pub m_read: UserReceipts,
 }