Correct issue link in design docs

[encukou]

Issue #23 was closed as a duplicate of #265.
Have the docs link to the open issue.

[codecov]

Codecov Report

Merging #388 into master will increase coverage by 3.39%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #388      +/-   ##
==========================================
+ Coverage   95.91%   99.31%   +3.39%     
==========================================
  Files          87       87              
  Lines       10334    10334              
  Branches      714      714              
==========================================
+ Hits         9912    10263     +351     
+ Misses        376       56     -320     
+ Partials       46       15      -31

Impacted Files	Coverage Δ
trio/tests/test_highlevel_open_tcp_listeners.py	100% <0%> (+0.7%)	⬆️
trio/tests/test_highlevel_socket.py	100% <0%> (+0.75%)	⬆️
trio/_core/_run.py	100% <0%> (+0.81%)	⬆️
trio/tests/test_ssl.py	100% <0%> (+0.9%)	⬆️
trio/tests/test_testing.py	100% <0%> (+1.46%)	⬆️
trio/tests/test_deprecate.py	100% <0%> (+1.56%)	⬆️
trio/_core/tests/test_result.py	100% <0%> (+2.08%)	⬆️
trio/_deprecate.py	100% <0%> (+2.73%)	⬆️
trio/_core/tests/test_windows.py	100% <0%> (+3.33%)	⬆️
trio/_core/_multierror.py	99.4% <0%> (+3.59%)	⬆️
... and 17 more

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[njsmith]

Thanks!