fix fractal_higuchi

[zen-juen]

Checklist

Here are some things to check before creating the PR. If you encounter any issues, do let us know :)

• I have read the CONTRIBUTING file.
• My PR is targetted at the dev branch (and not towards the master branch).
• I ran the CODE CHECKS on the files I added or modified and fixed the errors.
• I have added the newly added features to News.rst (if applicable)

[codecov-commenter]

Codecov Report

Merging #505 (6e62b0e) into dev (2ee3f4e) will increase coverage by 0.03%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##              dev     #505      +/-   ##
==========================================
+ Coverage   85.31%   85.34%   +0.03%     
==========================================
  Files         199      199              
  Lines        9736     9740       +4     
==========================================
+ Hits         8306     8313       +7     
+ Misses       1430     1427       -3     

Impacted Files	Coverage Δ
neurokit2/complexity/fractal_higuchi.py	95.40% <87.50%> (+1.14%)	⬆️
neurokit2/eda/eda_eventrelated.py	97.95% <0.00%> (-2.05%)	⬇️
neurokit2/complexity/fractal_dfa.py	93.24% <0.00%> (+0.18%)	⬆️
neurokit2/ecg/ecg_delineate.py	79.03% <0.00%> (+0.38%)	⬆️
neurokit2/rsp/rsp_simulate.py	99.10% <0.00%> (+0.89%)	⬆️

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