Fix markup used in the documentation of sys.prefix and sys.exec_prefix.

Fix markup used in the documentation of sys.prefix and sys.exec_prefix.

- Using the file role with {placeholders} is IMO clearer than fake
  Python code.

- The fact that sys.version[:3] gives '3.2' is a CPython detail and
  should not be advertised (see #9442), even if some stdlib modules
  currently rely on that detail.