masthead-followup: Add missing traling slash (#30681)

site/layouts/partials/home/masthead-followup.html

@@ -23,7 +23,7 @@ <h2 class="display-4">BootstrapCDN</h2>
   <p class="lead lead-lg mw-md-75 mx-auto">
     Use the <a href="https://www.bootstrapcdn.com/">BootstrapCDN</a> to deliver fast, cached, and compiled versions of Bootstrap’s CSS and JavaScript. No jQuery is required, but don't forget to include Popper.js for some components.
   </p>
-  <a class="btn btn-lg btn-outline-primary mb-4" href="/docs/{{ .Site.Params.docs_version }}/getting-started/introduction">Explore the docs</a>
+  <a class="btn btn-lg btn-outline-primary mb-4" href="/docs/{{ .Site.Params.docs_version }}/getting-started/introduction/">Explore the docs</a>
   <div class="text-left mx-md-5 px-md-5">
     <h5>CSS only</h5>
     {{ highlight (printf (`<link rel="stylesheet" href="%s" integrity="%s" crossorigin="anonymous">`) .Site.Params.cdn.css .Site.Params.cdn.css_hash) "html" "" }}