Remove ambiquity in what we are referring to in the documentation of config vs...

Remove ambiquity in what we are referring to in the documentation of config vs configuring the server itself

guides/source/api_app.md

@@ -289,7 +289,7 @@ You can learn more about how to use `Rack::Sendfile` with popular
 front-ends in [the Rack::Sendfile
 documentation](http://rubydoc.info/github/rack/rack/master/Rack/Sendfile).
 
-Here are some values for popular servers, once they are configured, to support
+Here are some values for this header for some popular servers, once these servers are configured to support
 accelerated file sending:
 
 ```ruby