fix link to CONTRIBUTING doc in issue template

[llimllib]

When I went to file issue #1050, I noticed that the link to the CONTRIBUTING doc was broken - it went to https://github.com/gboeing/CONTRIBUTING.md instead of https://github.com/gboeing/osmnx/CONTRIBUTING.md

Fix the link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.89%. Comparing base (0b2c24a) to head (ddddfda).
Report is 778 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1051   +/-   ##
=======================================
  Coverage   97.89%   97.89%           
=======================================
  Files          26       26           
  Lines        2475     2475           
=======================================
  Hits         2423     2423           
  Misses         52       52           

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