docs(configuration): set `allow_sign_up` to `true` for gerneric OAuth (#6928)

This enables Grafana to accept new users authenticated through OAuth, without this set to true only previously authenticated users may log in with OAuth - and thus defeating the purpose.

docs/sources/installation/configuration.md

@@ -394,7 +394,7 @@ browser to access Grafana, but with the prefix path of `/login/generic_oauth`.
     token_url =
     api_url =
     allowed_domains = mycompany.com mycompany.org
-    allow_sign_up = false
+    allow_sign_up = true
 
 Set api_url to the resource that returns basic user info.