Remove unused reference to 'shift' from the documentation

[ehrlinger]

This pull request makes a small change to the _pkgdown.yml configuration file by removing the shift function from the list of documented contents under the "reference" section.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.37%. Comparing base (96f0b97) to head (417e3b2).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #66   +/-   ##
=======================================
  Coverage   83.37%   83.37%           
=======================================
  Files          23       23           
  Lines        1816     1816           
=======================================
  Hits         1514     1514           
  Misses        302      302           

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