suggest edit of sources.list to enable deb-src's (#759)

* suggest edit of sources.list to enable deb-src's

When using WSL with Ubuntu, it was not obvious how to get the relevant deb-src to avoid error "E: You must put some 'deb-src' URIs in your sources.list".
The file already contained the urls which can be uncommented to continue the process of installin dependencies.

* Fix markup.

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

Author: dandiez

setup.rst

@@ -319,6 +319,10 @@ Take Ubuntu 22.04 LTS (Jammy Jellyfish) for example::
 
    deb-src http://archive.ubuntu.com/ubuntu/ jammy main
 
+Alternatively, uncomment lines with ``deb-src`` using an editor, e.g.::
+
+   sudo nano /etc/apt/sources.list
+
 For other distributions, like Debian, change the URL and names to correspond
 with the specific distribution.