docs: use correct openidconnect redirect URI
Created by: ggilmore
Fixes: https://github.com/sourcegraph/enterprise/issues/13515
<appUrl>/.auth/openidconnect/callback
-> <appUrl>/.auth/callback
Created by: ggilmore
Fixes: https://github.com/sourcegraph/enterprise/issues/13515
<appUrl>/.auth/openidconnect/callback
-> <appUrl>/.auth/callback