Wire up real Stripe sandbox test key and example book price ID

From the Two Squirrels Press Stripe sandbox account created and walked
through Managed Payments setup for. Both values are test-mode / safe
to commit; swap to live-mode equivalents at go-live.

content/books/example-book.md

@@ -3,7 +3,7 @@ title: "Example Book Title"
 author: "Author Name"
 cover_image: "https://downloads.twosquirrelspress.com/covers/example-book.jpg"
 description: "One-line jacket copy for listing pages."
-stripe_price_id: "price_XXXXXXXXXXXXXXXXXXXXXXXX"
+stripe_price_id: "price_1TtYspQEDYBEXYYF8YINz8y3"
 price_display: "$6.99"
 formats: ["EPUB", "PDF"]
 draft: true