[css-fonts-4] Fix markup for the production rule of <family-name>

[cdoublev]

This PR adds a .prod class name to the <pre> containing the production rule of <family-name>.

As commented in its code, w3c/reffy extracts a production rule if it is contained in a <pre class=prod> of if its left hand side is contained in a <dfn>:

<pre class=prod><foo> = bar</pre>
<pre><dfn><foo></dfn> = bar</pre>

[w3cbot]

svgeesus marked as non substantive for IPR from ash-nazg.