fix formatting of shell commands

"sudo zypper in python3-idle" gets highlighted as Python code. specifically "in" is highlighted in green. Since this is a shell command it needs to be formatted as such.

Author: partev

Doc/using/unix.rst

@@ -42,20 +42,28 @@ Installing IDLE
 
 In some cases, IDLE might not be included in your Python installation.
 
-* For Debian and Ubuntu users::
+* For Debian and Ubuntu users:
+
+.. code-block:: shell
 
    sudo apt update
    sudo apt install idle
 
-* For Fedora, RHEL, and CentOS users::
+* For Fedora, RHEL, and CentOS users:
+
+.. code-block:: shell
 
    sudo dnf install python3-idle
 
-* For SUSE and OpenSUSE users::
+* For SUSE and OpenSUSE users:
+
+.. code-block:: shell
 
    sudo zypper in python3-idle
 
-* For Alpine Linux users::
+* For Alpine Linux users:
+
+.. code-block:: shell
 
    sudo apk add python3-idle