host-header rewrite examples should be updated to use host header flag, instead of add headers

[justngrok]

Rewriting the Host header examples use the add header syntax, instead of our special case host-header flag.
https://ngrok.com/docs/http/?cty=agent-config#rewrite-host-header

Examples should be updated with the correct syntax.

Example:
Agent Config Example

tunnels:
  example:
    proto: http
    addr: 8080
    request_header:
      add: ["host: localhost"]

should be:

tunnels:
  example:
    proto: http
    addr: 8080
    host_header: localhost