fix(ui): usePreventLeave default message (#15042)

### What?

The default message in `usePreventLeave` is not structured correctly.

### Why?

The word `you` is missing from the sentence.

### How?

Adds the word `you` to the sentence.

Co-authored-by: Paul Popus <paul@payloadcms.com>

Author: rilrom

packages/ui/src/elements/LeaveWithoutSaving/usePreventLeave.tsx

@@ -59,7 +59,7 @@ export const useBeforeUnload = (enabled: (() => boolean) | boolean = true, messa
 
 export const usePreventLeave = ({
   hasAccepted = false,
-  message = 'Are you sure want to leave this page?',
+  message = 'Are you sure you want to leave this page?',
   onAccept,
   onPrevent,
   prevent = true,