Update README.md (#69)

Update the example code 
from: 
```ts
 if (ticket.id) {
    receiptIds.push(ticket.id);
  }
```
to 
```ts
 if (ticket.status == "ok") { // doing this help with typescript error
  receiptIds.push(ticket.id)
}
```
The typescript Error: 
```
Property 'id' does not exist on type 'ExpoPushTicket'.
Property 'id' does not exist on type 'ExpoPushErrorReceipt'.
```
this is due to the type being declare like this 
```ts
export type ExpoPushReceiptId = string;

export type ExpoPushSuccessTicket = {
  status: 'ok';
  id: ExpoPushReceiptId;
};

export type ExpoPushErrorTicket = ExpoPushErrorReceipt;

export type ExpoPushTicket = ExpoPushSuccessTicket | ExpoPushErrorTicket;
```
in `src/ExpoClient.ts`

README.md

@@ -88,7 +88,7 @@ let receiptIds = [];
 for (let ticket of tickets) {
   // NOTE: Not all tickets have IDs; for example, tickets for notifications
   // that could not be enqueued will have error information and no receipt ID.
-  if (ticket.id) {
+   if (ticket.status === 'ok') {
     receiptIds.push(ticket.id);
   }
 }