Fix spacing in deprecation warning

[Callek]

I was fixing some deprecation warnings in code I use, and came across this formatting error. (reads "valueis")

[codecov-io]

Codecov Report

Merging #525 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #525   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          14     14           
  Lines        3385   3385           
  Branches      235    235           
=====================================
  Hits         3385   3385

Impacted Files	Coverage Δ
arrow/arrow.py	100% <100%> (ø)	⬆️

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[Syeberman]

Thanks @Callek!