Fix variable names in docstring example for serve_tcp

[shuckc]

As discussed https://gitter.im/python-trio/general?at=5b702354a6af14730b25304e this is a small patch to example code in a doc string.

[codecov]

Codecov Report

Merging #597 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #597   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          91       91           
  Lines       10823    10823           
  Branches      753      753           
=======================================
  Hits        10749    10749           
  Misses         56       56           
  Partials       18       18

Impacted Files	Coverage Δ
trio/_highlevel_open_tcp_listeners.py	100% <ø> (ø)	⬆️

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[pquentin]

Thank you @shuckc! Merged in master.

[njsmith]

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[njsmith]

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