fix: Fork me on GitHub update broken link

point to github blog initial posting, which is live:

https://github.blog/2008-12-19-github-ribbons/

Author: joslloand

_includes/header.html

@@ -1,6 +1,6 @@
   <!-- Fork me at GitHub -->
 
-  <a href="https://github.com/ambisonictoolkit"><img style="position: absolute; top: 50px; right: 0; border: 0;" src="https://camo.githubusercontent.com/365986a132ccd6a44c23a9169022c0b5c890c387/68747470733a2f2f73332e616d617a6f6e6177732e636f6d2f6769746875622f726962626f6e732f666f726b6d655f72696768745f7265645f6161303030302e706e67" alt="Fork me on GitHub" data-canonical-src="https://s3.amazonaws.com/github/ribbons/forkme_right_red_aa0000.png"></a>
+  <a href="https://github.com/ambisonictoolkit"><img style="position: absolute; top: 50px; right: 0; border: 0;" src="https://github.blog/wp-content/uploads/2008/12/forkme_right_red_aa0000.png?resize=149%2C149" class="attachment-full size-full" alt="Fork me on GitHub" data-recalc-dims="1"></a>
 
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