[fix] doc of setting server.base_url, env is SEARXNG_BASE_URL

Related:

- https://github.com/searxng/searxng/issues/4634

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/admin/settings/settings_server.rst

@@ -20,7 +20,7 @@
          X-Robots-Tag : noindex, nofollow
          Referrer-Policy : no-referrer
 
-``base_url`` : ``$SEARXNG_URL``
+``base_url`` : ``$SEARXNG_BASE_URL``
   The base URL where SearXNG is deployed.  Used to create correct inbound links.
 
 ``port`` & ``bind_address``: ``$SEARXNG_PORT`` & ``$SEARXNG_BIND_ADDRESS``