MAINT: Add single quote via !r for f-string

[Zeroto521]

• closes #xxxx
• whatsnew entry

A single quote could enhance it is the variable in a warning message or error message.

[codecov]

Codecov Report

Merging #520 (67ad031) into main (39608ce) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #520   +/-   ##
=======================================
  Coverage   99.63%   99.63%           
=======================================
  Files          60       60           
  Lines         828      828           
  Branches       92       92           
=======================================
  Hits          825      825           
  Misses          1        1           
  Partials        2        2           

Impacted Files	Coverage Δ
dtoolkit/accessor/_util.py	100.00% <ø> (ø)
dtoolkit/transformer/_validation.py	100.00% <ø> (ø)
dtoolkit/util/generic/multi_if_else.py	100.00% <ø> (ø)
dtoolkit/accessor/dataframe/to_series.py	100.00% <100.00%> (ø)

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