docs: fix port used in connection for local-testing.md

[dobsonj]

This section of the doc was slightly confusing at first glance. The doc says:

Connect to the LOCAL_PORT:

nc 127.0.0.1 [external_port]

But it really should be LOCAL_PORT. For example:

> kctf chal debug port-forward &
[1] 21137
[*] starting port-forward, ctrl+c to exit
Forwarding from 127.0.0.1:43373 -> 1337

> nc localhost 43373
== proof-of-work: disabled ==
CTF{TestFlag}

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