Replace finished method with onFinish

The appropriate name of the post-authentication callback seems to be `onFinish`, not `finished` based on the code here: https://github.com/blockstack/connect/blob/main/packages/connect/src/auth.ts

Author: kezike

src/pages/build-apps/guides/authentication.md

@@ -114,7 +114,7 @@ function authenticate() {
 The `showConnect` function accepts a number of properties within a parameter object such as:
 
 - The app's `name` and `icon`: provided as strings comprising the `appDetails` object property.
-- The `redirectTo` string: used to provide a URL to which the user should be redirected upon successful authentication. The `finished` callback serves a similar purpose by handling successful authentication within a context of a popup window.
+- The `redirectTo` string: used to provide a URL to which the user should be redirected upon successful authentication. The `onFinish` callback serves a similar purpose by handling successful authentication within a context of a popup window.
 - The `userSession` object initiated above.
 
 Once the user selects the button presented in this modal, they are passed to the Stacks Wallet for authenticator with the `authRequest` token as a GET parameter. From there they can confirm authentication and generate a new _Secret Key_ or Stacks identity before doing so, as needed before coming back to the app.
@@ -150,7 +150,7 @@ The authenticated state can later be detected by the `isUserSignedIn` method in
 
 If the user has indeed confirmed authentication in the context of a popup window, the authenticator will resolve the pending authentication state automatically with the app within the parent window.
 
-It will then trigger the `finished` function provided above, which can be used similarly to save the user's information into their session as retrieved with `userSession.loadUserData()`.
+It will then trigger the `onFinish` function provided above, which can be used similarly to save the user's information into their session as retrieved with `userSession.loadUserData()`.
 
 ## Usage in React Apps