README: build against caddy v2.6.4

Signed-off-by: Will Norris <will@tailscale.com>

Author: willnorris

README.md

@@ -20,7 +20,7 @@ Use [xcaddy](https://github.com/caddyserver/xcaddy) to build Caddy with the
 Tailscale plugin included.
 
 ```
-xcaddy build master --with github.com/tailscale/caddy-tailscale
+xcaddy build v2.6.4 --with github.com/tailscale/caddy-tailscale
 ```
 
 ## Caddy network listener