Editorial: remove prefetch-src

It was removed by CSP: w3c/webappsec-csp#582.

Author: noamr

fetch.bs

@@ -1814,7 +1814,7 @@ not always relevant and might require different behavior.
   <tr>
    <td>"<code>prefetch</code>"
    <td rowspan=2>""
-   <td rowspan=2><code>prefetch-src</code>
+   <td rowspan=2><code>default-src</code> (no specific directive)
    <td>HTML's <code>&lt;link rel=prefetch></code>
   <tr>
    <td>"<code>prerender</code>"