Change "May 3rd" -> "May 3" in PromCon banner

Signed-off-by: Julius Volz <[email protected]>

Author: juliusv

content/index.html

@@ -13,7 +13,7 @@ <h1>From metrics to insight</h1>
 </div>
 
 <div class="newsbar">
-  Join <a href="https://promcon.io/2021-online/">PromCon Online 2021</a>, the Prometheus users conference, on May 3rd, 2021.</a>
+  Join <a href="https://promcon.io/2021-online/">PromCon Online 2021</a>, the Prometheus users conference, on May 3, 2021.</a>
 </div>
 
 <div class="container">