README: remove link to unofficial iperf website

Ubuntu packages iperf3, there's no need to get the outdated binaries
offered by iperf.fr.
See also esnet/iperf#1459 (comment)

README.md

@@ -48,7 +48,8 @@ Install the following on both the WG server and WG peer
     ```
 * Install `iperf3`
     ```bash
-    # Source: https://iperf.fr/iperf-download.php
+    # Ubuntu
+    sudo apt install iperf3
     ```
 * Install `sed`
     ```bash