Update test-standard link in contributing docs

[gliptak]

correcting

arning, treated as error:
/home/runner/work/scrapy/scrapy/docs/contributing.rst:216:unknown document: twisted:core/development/policy/test-standard
ERROR: InvocationError for command /home/runner/work/scrapy/scrapy/.tox/docs/bin/sphinx-build -W -b html . /home/runner/work/scrapy/scrapy/.tox/docs/tmp/html (exited with code 2)
___________________________________ summary ____________________________________
ERROR:   docs: commands failed

[codecov]

Codecov Report

Merging #5631 (0222227) into master (681d114) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5631   +/-   ##
=======================================
  Coverage   88.66%   88.66%           
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  Files         162      162           
  Lines       10965    10965           
  Branches     1894     1894           
=======================================
  Hits         9722     9722           
  Misses        963      963           
  Partials      280      280           

[wRAR]

The test now says

/home/runner/work/scrapy/scrapy/docs/contributing.rst:216:unknown document: twisted:core/development/test-standard

[gliptak]

@wRAR this seem to be the page available https://docs.twisted.org/en/stable/development/test-standard.html

maybe cross linkage works differently

[Gallaecio]

I think we may need to remove core/ as well.

[gliptak]

Thanks @Gallaecio updated

[Gallaecio]

Thanks!