Improve highlighting of some code blocks. (GH-6401)

diff --git a/Doc/using/unix.rst b/Doc/using/unix.rst
index ac99b69..8b392f8 100644
--- a/Doc/using/unix.rst
+++ b/Doc/using/unix.rst
@@ -118,7 +118,9 @@
 =============
 
 To easily use Python scripts on Unix, you need to make them executable,
-e.g. with ::
+e.g. with
+
+.. code-block:: shell-session
 
    $ chmod +x script