Add git fetch upstream to contributing doc.

[carlosgmartin]

In the contributing doc, change

git rebase upstream/main

to

git fetch upstream
git rebase upstream/main

This ensures that users fetch upstream before rebasing. I had to do this for #3712 and prior PRs. It also matches JAX's contributing doc.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.70%. Comparing base (3da44bc) to head (84809ef).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3757   +/-   ##
=======================================
  Coverage   58.70%   58.70%           
=======================================
  Files         102      102           
  Lines       12412    12412           
=======================================
  Hits         7286     7286           
  Misses       5126     5126           

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[cgarciae]

Thanks @carlosgmartin !