Fixed contributing link

[jacobtomlinson]

Closes #1816

[codecov]

Codecov Report

Merging #1818 (1adeeaa) into master (d346323) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1818   +/-   ##
=======================================
  Coverage   99.29%   99.29%           
=======================================
  Files          80       80           
  Lines        4697     4697           
=======================================
  Hits         4664     4664           
  Misses         33       33           

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