style: change sponsor button from warning to secondary color

Reduce visual distraction from the primary "Join VanRuby" call-to-action by changing the "Become a sponsor" button from gold (btn-warning) to gray (btn-secondary).

Author: mrleeio

index.html

@@ -44,7 +44,7 @@ <h5 class="card-title">Engaging Speakers</h5>
         <h5 class="card-title">Community Driven</h5>
         <p class="card-text">VanRuby would not exist without support from our sponsors. If your school or business could
           benefit from connecting with our community, we would love to hear from you.</p>
-        <a href="{{ site.baseurl }}/sponsors.html" class="btn btn-warning mt-auto">Become a sponsor</a>
+        <a href="{{ site.baseurl }}/sponsors.html" class="btn btn-secondary mt-auto">Become a sponsor</a>
         <p class="text-center mt-3 mb-0">
           <small>No GitHub account? <a href="mailto:[email protected],[email protected]?subject=VanRuby Sponsorship">Email us instead</a></small>
         </p>