docs(mtls) use number for verify depth (#913)

Use an arbitrary number (3) for the verify depth. This is fine for the
vast majority of certificate chains. The default (1) is a bit low, and
causes issues with multiple intermediates. The current value, "on", is
definitely wrong, as this setting only accepts a number.

docs/guides/upstream-mtls.md

@@ -33,7 +33,7 @@ environment variables on Kong's container in your deployment:
 
 ```
 KONG_NGINX_PROXY_PROXY_SSL_VERIFY="on"
-KONG_NGINX_PROXY_PROXY_SSL_VERIFY_DEPTH="on"
+KONG_NGINX_PROXY_PROXY_SSL_VERIFY_DEPTH="3"
 KONG_NGINX_PROXY_PROXY_SSL_TRUSTED_CERTIFICATE="/path/to/ca_certs.pem"
 ```