config: update example to new mechanism

README.adoc

@@ -11257,11 +11257,19 @@ It gets annoying to retype `--arch aarch64` for every single command, or to reme
 So simplify that, do:
 
 ....
-cp config.example data/config
+cp config.py data/
 ....
 
 and then edit the `data/config` file to your needs.
 
+Source: link:config.py[]
+
+You can also choose a different configuration file explicitly with:
+
+....
+./run --config data/config2.py
+....
+
 === Build the documentation
 
 You don't need to depend on GitHub:

config.example

@@ -1,4 +0,0 @@
-#!/usr/bin/env python
-arch = 'aarch64'
-gem5 = True
-run_id = 'asdf'

config.py

@@ -0,0 +1,3 @@
+def set_args(args):
+    args['arch'] = 'aarch64'
+    args['gem5'] = True