fix(docs): fix reference to master branch; use mainnet instead

[danceratopz]

🗒️ Description

Fixes references to master branch in the EIP Author Manual. We use mainnet now.

🔗 Related Issues or PRs

N/A.

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  uvx tox -e static

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.

Cute Animal Picture

[spencer-tb]

LGTM! Thanks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.07%. Comparing base (05117e5) to head (b6f9dff).
⚠️ Report is 3 commits behind head on forks/amsterdam.

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #2037   +/-   ##
================================================
  Coverage            86.07%   86.07%           
================================================
  Files                  599      599           
  Lines                39527    39527           
  Branches              3780     3780           
================================================
  Hits                 34021    34021           
  Misses                4872     4872           
  Partials               634      634           

Flag	Coverage Δ
unittests	86.07% <ø> (ø)

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