Wording fix for docs/running.html

[JeremyVisser]

In ​How To Start Tahoe-LAFS, there is the following:

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If your node is behind a firewall or NAT device and want other clients to connect to it then you'll need to open a port in your firewall. To do that you'll need to know which port tahoe is listening on as, by default, it listens on an arbitrary port number.

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"arbitrary port number" doesn't explain much here. Zooko explained that tahoe picks a random port number, and then stores it in the client.port file. There is no harm in saying this explicitly, like so:

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To do that you'll need to know which port tahoe is listening on. If you haven't already set a port number, tahoe picks a random port to listen on (and remembers this number in the client.port file).

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[zooko]

applied in 6cdf68785b9d208e Thanks!