Change columns for downloads section on medium screens (#5400)

Author: Mykola Ptushchuk

templates/index.html

@@ -148,10 +148,10 @@ <h2>Guidelines</h2>
 
 <div class="p-strip">
   <div class="row">
-    <div class="col-4 col-medium-2">
+    <div class="col-4">
       <h2>Downloads</h2>
     </div>
-    <div class="p-card--highlighted col-4 col-medium-2">
+    <div class="p-card--highlighted col-4 col-medium-3">
       <div class="p-heading-icon--small">
         <div class="p-heading-icon__header">
           <img class="p-heading-icon__img" src="https://assets.ubuntu.com/v1/0276b842-github-icon.svg" alt="GitHub repo:" />
@@ -162,7 +162,7 @@ <h3 class="p-heading-icon__title">Vanilla Framework</h3>
         <small>See the <a href="https://github.com/canonical/vanilla-framework/releases">release notes</a> for details on the latest&nbsp;updates.</small>
       </div>
     </div>
-    <div class="p-card--highlighted col-4 col-medium-2">
+    <div class="p-card--highlighted col-4 col-medium-3">
       <div class="p-heading-icon--small">
         <div class="p-heading-icon__header">
           <img class="p-heading-icon__img" src="https://assets.ubuntu.com/v1/9a5b9c51-sketch-icon.svg" alt="Sketch library:" />