Doc: Fix variable name in docs.
Doc: Fix variable name in docs.

The variable used in cli/environment.lua is OPENTRACING_FORWARD_HEADER,
and the docs were incorrect.

Signed-off-by: Eloy Coto <2a8acc28452926ceac4d9db555411743254cf5f9@gmail.com>

3scale/docker-gateway,3scale/docker-gateway