Add missing link in ui.sub_pages demo (#5504)

### Motivation

In #5498 we noticed that the displayed code doesn't produce the exact
same output as the online demo.
https://nicegui.io/documentation/sub_pages#nested_sub_pages

### Implementation

This PR adds the missing link.

### Progress

- [x] I chose a meaningful title that completes the sentence: "If
applied, this PR will..."
- [x] The implementation is complete.
- [x] Pytests are not necessary.
- [x] Documentation is not necessary.

Author: falkoschindler

website/documentation/content/sub_pages_documentation.py

@@ -289,6 +289,7 @@ def main():
 
     def other():
         ui.label('sub page')
+        # ui.link('Go to main', '/')
         # ui.link('Go to A', '/other/a')
         # ui.link('Go to B', '/other/b')
         # ui.sub_pages({