net: report port number correctly in Plan 9 error

net: report port number correctly in Plan 9 error

The code was incorrectly using a string conversion of a numeric port
to display the port number.

No test because as far as I can tell this code is only executed if
there is some error in a /net file.

Updates #32479

Change-Id: I0b8deebbf3c0b7cb1e1eee0fd059505f3f4c1623
Reviewed-on: https://go-review.googlesource.com/c/go/+/221377
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>