Added Mint 21 on download page.

Author: Kasper Peeters

web2/cadabra2/source/download.html

@@ -63,6 +63,7 @@ <h2>Binary packages</h2>
 	 <img src="static/images/mintlogo.png"/>
 	 <div class="distronum"><a href="https://cadabra.science/packages/mint19/">Mint Tara 19</a></div>
 	 <div class="distronum"><a href="https://cadabra.science/packages/mint20/">Mint Ulyana 20</a></div>
+     <div class="distronum"><a href="https://cadabra.science/packages/mint21/">Mint Vanessa 21</a></div>
   </div>
 
 <!--  <div class="distrotype">