Fix string unmap with Parser1

[regadas]

Just noticed that for string it unmap'ing 2 times. not sure if I'm missing anything.

[codecov-io]

Codecov Report

Merging #122 (d682675) into main (15d42dd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #122   +/-   ##
=======================================
  Coverage   94.59%   94.59%           
=======================================
  Files           6        6           
  Lines         758      758           
  Branches       77       77           
=======================================
  Hits          717      717           
  Misses         41       41           

Impacted Files	Coverage Δ
core/shared/src/main/scala/cats/parse/Parser.scala	95.05% <100.00%> (ø)

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[johnynek]

Thanks!