no need to specify caddy adapter

caddy 2.8.4 fixed an issue with detecting caddyfiles, so we no longer
need to explicitly specify the adapter type, since all of our config
files end in `.caddyfile`.

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

README.md

@@ -47,8 +47,7 @@ See the comments in the individual files for details.
 Run them with:
 
 ```
-TS_AUTHKEY=<tskey-auth-XXXXX> \
-./caddy run -a caddyfile -c examples/<file>
+TS_AUTHKEY=<tskey-auth-XXXXX> ./caddy run -c examples/<file>
 ```
 
 [examples directory]: ./examples/