DOC : correct URL for black tool in CONTRIBUTING.rst

[Turiok]

Hi,

I try to add some changes but one URL in the contributing file was wrong.
Probably a change from black maintainers.

[codecov-commenter]

Codecov Report

Merging #946 (f302f5b) into master (64e86be) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #946   +/-   ##
=======================================
  Coverage   36.50%   36.50%           
=======================================
  Files          34       34           
  Lines        6527     6527           
  Branches     1007     1007           
=======================================
  Hits         2383     2383           
  Misses       4037     4037           
  Partials      107      107           

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[gonchik]

Hi!
Thanks