docs: remove outdated reference to doc.useServiceAccountAuth()

Author: Theo Ephraim

docs/guides/authentication.md

@@ -134,9 +134,10 @@ Here's one way to get it to work (using heroku cli):
 
 And in some other scenarios, replacing newlines in your code can be helpful, for example:
 ```js
-await doc.useServiceAccountAuth({
- client_email: process.env.GOOGLE_SERVICE_ACCOUNT_EMAIL,
- private_key: process.env.GOOGLE_PRIVATE_KEY.replace(/\\n/g, "\n"),
+const jwtFromEnv = new JWT({
+ email: process.env.GOOGLE_SERVICE_ACCOUNT_EMAIL,
+ key: process.env.GOOGLE_PRIVATE_KEY.replace(/\\n/g, "\n"),
+ scopes: SCOPES,
 });
 ```