Update the node join instruction to reference "username"

related to https://github.com/juanfont/headscale/pull/1144

Signed-off-by: Christian Heusel <christian@heusel.eu>

api.go

@@ -78,7 +78,7 @@ var registerWebAPITemplate = template.Must(
 		<p>
 			Run the command below in the headscale server to add this machine to your network:
 		</p>
-		<pre><code>headscale -n NAMESPACE nodes register --key {{.Key}}</code></pre>
+		<pre><code>headscale nodes register --user USERNAME --key {{.Key}}</code></pre>
 	</body>
 </html>
 `))