Update configuration.md

Mention replacing `.` with `_` with environment variable override.

docs/sources/installation/configuration.md

@@ -28,14 +28,19 @@ using environment variables using the syntax:
     GF_<SectionName>_<KeyName>
 
 Where the section name is the text within the brackets. Everything
-should be upper case. For example, given this configuration setting:
+should be upper case, `.` should be replaced by `_`. For example, given these configuration settings:
 
     [security]
     admin_user = admin
     
+    [auth.google]
+    client_secret = 0ldS3cretKey
+    
+
 Then you can override that using:
 
     export GF_SECURITY_ADMIN_USER=true
+    export GF_AUTH_GOOGLE_CLIENT_SECRET=newS3cretKey
 
 <hr>