Add filenames to code snippets in hello_tutorial

Fixes #1369

repos/hello_tutorial/doc/hello_tutorial.txt

@@ -117,6 +117,7 @@ Writing server code
 ###################
 
 Now let's write a server providing the interface defined by _Hello::Session_.
+We will put all of this code in 'src/hello/server/main.cc'
 
 
 Implementing the server side
@@ -331,6 +332,8 @@ Afterwards, we create a 'Hello::Session_client' object with it and invoke calls.
 addition, we use the Timer service that comes with Genode. This server
 enables us to sleep for a certain amount of milliseconds.
 
+Put this code into 'src/hello/client/main.cc':
+
 !#include <base/env.h>
 !#include <base/printf.h>
 !#include <hello_session/client.h>