Update the Gecko Plugin API Reference link (redirected)

A few problems with the page were fixed in
https://bugzilla.mozilla.org/show_bug.cgi?id=1244121

Part of #573

source

@@ -116246,7 +116246,7 @@ INSERT INTERFACES HERE
    <dd><cite><a href="https://drafts.csswg.org/mediaqueries/">Media Queries</a></cite>, H. Lie, T. &Ccedil;elik, D. Glazman, A. van Kesteren. W3C.</dd>
 
    <dt id="refsNPAPI">[NPAPI]</dt>
-   <dd>(Non-normative) <cite><a href="https://developer.mozilla.org/en-US/Add-ons/Plugins/Gecko_Plugin_API_Reference">Gecko Plugin API Reference</a></cite>. Mozilla.</dd>
+   <dd>(Non-normative) <cite><a href="https://developer.mozilla.org/en-US/docs/Plugins/Guide">Gecko Plugin API Reference</a></cite>. Mozilla.</dd>
 
    <dt id="refsNPN">[NPN]</dt>
    <dd><cite><a href="https://tools.ietf.org/html/draft-agl-tls-nextprotoneg">Transport Layer Security (TLS) Next Protocol Negotiation Extension</a></cite>, A. Langley. IETF.</dd>