Inconsistencies in keep-alive advice

[thelangley]

Guidance in the docs appears to indicate keep-alive on the gorouter is disabled by default.

original advice

5 years ago it was enabled by default

I think the docs have grown organically over time so just a tiny tweak to indicate that it's already enabled by default would be ace

https://github.com/cloudfoundry/docs-cf-admin/blob/master/routing-keepalive.html.md.erb#L6