decorators.md: quote "Bar" forward reference

[the9000]

The point of forward references is that they are strings. Adding the forgotten quotes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.91%. Comparing base (e191403) to head (535c7f6).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #650   +/-   ##
=======================================
  Coverage   90.91%   90.91%           
=======================================
  Files          13       13           
  Lines        2004     2004           
  Branches      367      367           
=======================================
  Hits         1822     1822           
  Misses        123      123           
  Partials       59       59           

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