Fix `getKey` typo from PushSubscription example

[Sonna]

Fix a typo in the Push Subscription example that demonstrates how to manually create the same result from pushSubscription object with JSON.stringify by incorrectly calling getKeys instead of the correct getKey function name from pushSubscription object.

References:

• https://developer.mozilla.org/en-US/docs/Web/API/PushSubscription/getKey
• https://web-push-book.gauntface.com/chapter-02/01-subscribing-a-user/#send-a-subscription-to-your-server

[Sonna]

Closing PR since Project appears to be inactive