docs: purchases.md use object instead of params in requestSubscription call

[frothAml]

The example shown in purchases.md has the wrong interface use for the function requestSubscription

In the docs example is shown as

      await requestSubscription(
        {sku},
        ...(offerToken && {subscriptionOffers: [{sku, offerToken}]}),
      );

While the interface to the function requires an object of the type RequestSubscription and not 2 args

[hyochan]

LGTM!