docs: Specify preferred Python string formatting technique

Author: Mason Simon

test/functional/README.md

@@ -28,6 +28,7 @@ don't have test cases for.
 - When subclassing the BitcoinTestFramwork, place overrides for the
   `set_test_params()`, `add_options()` and `setup_xxxx()` methods at the top of
   the subclass, then locally-defined helper methods, then the `run_test()` method.
+- Use `'{}'.format(x)` for string formatting, not `'%s' % x`.
 
 #### Naming guidelines