docs(proxy): update `sub_filter` for subfolder (#3046) [skip ci]

The updated `next.js` dependency has a new regex match in the code for `/^\/_next\/data\//`.

This `sub_filter` creates invalid regex `/^\/overseerr/_next\/data\//`
```nginx
    sub_filter '/_next' '/$app/_next';
```
It needs to be updated to substitute the correct the regex `/^\/overseerr\/_next\/data\//`
```nginx
    sub_filter '\/_next' '\/$app\/_next';
    sub_filter '/_next' '/$app/_next';
```

docs/extending-overseerr/reverse-proxy.md

@@ -138,6 +138,7 @@ location ^~ /overseerr {
     sub_filter 'href="/"' 'href="/$app"';
     sub_filter 'href="/login"' 'href="/$app/login"';
     sub_filter 'href:"/"' 'href:"/$app"';
+    sub_filter '\/_next' '\/$app\/_next';
     sub_filter '/_next' '/$app/_next';
     sub_filter '/api/v1' '/$app/api/v1';
     sub_filter '/login/plex/loading' '/$app/login/plex/loading';