Fix README image link (to master)

[justinpickering]

Context:

• Light mode README hero image currently points to the update-readme-image branch, it should point to master

Description of the Change:

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[tarik-elkhateeb]

Thanks @justinpickering

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.64%. Comparing base (6b20437) to head (c793cda).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6682   +/-   ##
=======================================
  Coverage   99.64%   99.64%           
=======================================
  Files         467      467           
  Lines       44183    44183           
=======================================
  Hits        44025    44025           
  Misses        158      158           

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