Fix broken contributors graph link

[mattphillips]

Summary

Currently the link in the contributors image points to: https://github.com/facebook/jest/blob/master/graphs/contributors which is a 404.

This PR updates the link to be absolute and points it to https://github.com/facebook/jest/graphs/contributors

[thymikee]

woo!

[codecov-io]

Codecov Report

Merging #6711 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6711   +/-   ##
=======================================
  Coverage   63.68%   63.68%           
=======================================
  Files         235      235           
  Lines        9003     9003           
  Branches        3        4    +1     
=======================================
  Hits         5734     5734           
  Misses       3268     3268           
  Partials        1        1

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[github-actions]

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