Fix 404 broken link for `cgi` module.

[codecov-io]

Codecov Report

Merging #127 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #127   +/-   ##
=======================================
  Coverage   95.81%   95.81%           
=======================================
  Files          18       18           
  Lines        1817     1817           
  Branches      274      274           
=======================================
  Hits         1741     1741           
  Misses         19       19           
  Partials       57       57

Impacted Files	Coverage Δ
webapp2_extras/xsrf.py	93.44% <ø> (ø)	:white_check_mark:
webapp2_extras/sessions.py	96.37% <100%> (ø)	:white_check_mark:
webapp2.py	95.47% <100%> (ø)	:white_check_mark:
webapp2_extras/json.py	100% <100%> (ø)	:white_check_mark:
webapp2_extras/auth.py	96.85% <100%> (ø)	:white_check_mark:

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[theacodes]

@toanant Thanks!