Fix a typo

[davidanthoff]

Fixes #54.

[codecov-io]

Codecov Report

Merging #55 into master will decrease coverage by 1.39%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master      #55     +/-   ##
=========================================
- Coverage    80.7%   79.31%   -1.4%     
=========================================
  Files           2        2             
  Lines         114      116      +2     
=========================================
  Hits           92       92             
- Misses         22       24      +2

Impacted Files	Coverage Δ
src/C_interface.jl	84.21% <0%> (ø)	⬆️
src/ReadStat.jl	78.35% <0%> (-1.65%)	⬇️

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