Update current tag on Headless CMS page (#848)

* Update current tag

copy below the "Headless CMS" tag from its current state ("A List of Content Management Systems for Jamstack Sites") to "A list of headless content management systems for Jamstack sites."
https://github.com/netlify/marketing/issues/1911

* Syntax change

---------

Co-authored-by: Sam <[email protected]>

Author: tferris22

src/site/headless-cms.njk

@@ -8,7 +8,7 @@ layout: layouts/base.njk
 <section class="md:mt-20">
   <h1 class="text-center">Headless CMS</h1>
   <p class="text-center">
-    A List of Content Management Systems for Jamstack Sites
+    A list of <a href="https://www.netlify.com/blog/complete-guide-to-headless-cms/" target="_blank" rel="noopener">headless content management systems</a> for Jamstack sites
   </p>
 </section>