Update dead link of plugin doc

The link to "Gecko Plugin API reference" page is no longer working. The latest one seems to be 
https://developer.mozilla.org/en-US/docs/Plugins/Guide

Update the documentation to use the new URL

Change-Id: Ic75f927e3c607696d752bb365f33dc9531897238
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3212946
Auto-Submit: Owen Min <[email protected]>
Commit-Queue: Thomas Anderson <[email protected]>
Reviewed-by: Thomas Anderson <[email protected]>
Cr-Commit-Position: refs/heads/main@{#929310}
diff --git a/docs/linux/plugins.md b/docs/linux/plugins.md
index 6c1eae60..5ed9dd6 100644
--- a/docs/linux/plugins.md
+++ b/docs/linux/plugins.md
@@ -4,7 +4,7 @@
 
 ### Plugins in general
 
-*   [Gecko Plugin API reference](https://developer.mozilla.org/en/Gecko_Plugin_API_Reference)
+*   [Gecko Plugin API reference](https://developer.mozilla.org/en-US/docs/Plugins/Guide)
     -- most important to read
 *   [Mozilla plugins site](http://www.mozilla.org/projects/plugins/)
 *   [XEmbed extension](https://developer.mozilla.org/en/XEmbed_Extension_for_Mozilla_Plugins)