fix: payload and secret are translated in some langs(use inline code)

The translated text in some languages like Japanese is not the same
as the original.
In this context, the text is payload and secret to derive the
signature.
So identification is important.

content/webhooks/using-webhooks/validating-webhook-deliveries.md

@@ -57,8 +57,8 @@ There are a few important things to keep in mind when validating webhook payload
 
 You can use the following `secret` and `payload` values to verify that your implementation is correct:
 
-* `secret`: "It's a Secret to Everybody"
-* `payload`: "Hello, World!"
+* `secret`: `It's a Secret to Everybody`
+* `payload`: `Hello, World!`
 
 If your implementation is correct, the signatures that you generate should match the following signature values: