docs: Correct event handler name in handling-updates.md

[linusnorton]

According to the TypeScript types and the docs https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onInstalled the method name should be addListener

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.77%. Comparing base (ea5b81d) to head (3a20523).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #653      +/-   ##
==========================================
- Coverage   86.94%   86.77%   -0.18%     
==========================================
  Files         114      114              
  Lines        9215     9215              
  Branches      921      924       +3     
==========================================
- Hits         8012     7996      -16     
- Misses       1189     1205      +16     
  Partials       14       14              

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