cmd/nginx-auth: update Expected-Tailnet documentation (#6055)

Signed-off-by: Xe Iaso <xe@tailscale.com>

cmd/nginx-auth/README.md

@@ -129,7 +129,7 @@ the `Expected-Tailnet` header to your auth request:
 ```nginx
 location /auth {
   # ...
-  proxy_set_header Expected-Tailnet "tailscale.com";
+  proxy_set_header Expected-Tailnet "tailnet012345.ts.net";
 }
 ```
 
@@ -146,6 +146,8 @@ generic "forbidden" error page:
 </html>
 ```
 
+You can get the tailnet name from [the admin panel](https://login.tailscale.com/admin/dns).
+
 ## Building
 
 Install `cmd/mkpkg`: