Update line number in diff.rb

Looking at the output from git diff on this level, it seems the correct line number is 23…
```diff
diff --git a/app.rb b/app.rb
index 4f703ca..3bfa839 100644
--- a/app.rb
+++ b/app.rb
@@ -23,7 +23,7 @@ get '/yet_another' do
   erb :success
 end
 get '/another_page' do
-  @message = get_response('data.json')
+  @message = get_response('server.json')
   erb :another
 end
```