docs: Update code snippets to use errors package for type assertions

[zyfy29]

document update for #3699

[gmlewis]

Thank you, @zyfy29!
LGTM.
Merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.11%. Comparing base (46f1bf2) to head (2e53530).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3746   +/-   ##
=======================================
  Coverage   91.11%   91.11%           
=======================================
  Files         187      187           
  Lines       16702    16702           
=======================================
  Hits        15218    15218           
  Misses       1296     1296           
  Partials      188      188           

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