Fix well class names in migration section of getting-started.html

getting-started.html

@@ -545,7 +545,7 @@ bootstrap/
         </tr>
         <tr>
           <td>Well sizes</td>
-          <td><code>.well-small</code> <code>.well-large</code></td>
+          <td><code>.well-sm</code> <code>.well-lg</code></td>
         </tr>
         <tr>
           <td>Alert links</td>