Change 'checkouted' to 'checked out'

[jellebouwman]

Small typo I found in one of the exception messages while testing some stuff in GTO.

[codecov-commenter]

Codecov Report

Merging #176 (aac20a5) into main (1ce252e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #176   +/-   ##
=======================================
  Coverage   81.64%   81.64%           
=======================================
  Files          16       16           
  Lines        1487     1487           
=======================================
  Hits         1214     1214           
  Misses        273      273           

Impacted Files	Coverage Δ
gto/exceptions.py	68.22% <100.00%> (ø)

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