Fix link to stage 4 PR in README

[trflynn89]

The lack of a scheme in the URL made my browser try to open:
https://github.com/tc39/proposal-temporal/blob/main/github.com/tc39/ecma262/pull/3759

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.83%. Comparing base (06d3e08) to head (1bdfbd3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3297   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files          22       22           
  Lines       10725    10725           
  Branches     1856     1856           
=======================================
  Hits        10493    10493           
  Misses        215      215           
  Partials       17       17           

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[ptomato]

Thanks!