[Fix] docs build warning

[danibene]

Description

This PR aims to address a warning in the docs check caused by ecg_delineate().

Proposed Changes

See #897 (comment)

Checklist

• I have read the CONTRIBUTING file.
• My PR is targeted at the dev branch (and not towards the master branch).

[codecov-commenter]

Codecov Report

Patch coverage: 58.06% and project coverage change: +0.02% 🎉

Comparison is base (d867214) 55.19% compared to head (65581dd) 55.22%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #898      +/-   ##
==========================================
+ Coverage   55.19%   55.22%   +0.02%     
==========================================
  Files         302      302              
  Lines       14142    14143       +1     
==========================================
+ Hits         7806     7810       +4     
+ Misses       6336     6333       -3     

Files Changed	Coverage Δ
neurokit2/complexity/entropy_spectral.py	25.00% <0.00%> (ø)
neurokit2/complexity/fractal_psdslope.py	17.64% <0.00%> (ø)
neurokit2/ecg/ecg_phase.py	85.29% <ø> (ø)
neurokit2/misc/progress_bar.py	14.28% <ø> (ø)
neurokit2/signal/signal_fillmissing.py	28.57% <0.00%> (ø)
neurokit2/ecg/ecg_quality.py	26.31% <30.00%> (ø)
neurokit2/stats/standardize.py	65.85% <33.33%> (ø)
neurokit2/ecg/ecg_delineate.py	80.56% <100.00%> (ø)
neurokit2/emg/emg_activation.py	90.14% <100.00%> (ø)
neurokit2/emg/emg_plot.py	88.15% <100.00%> (ø)
... and 3 more

... and 2 files with indirect coverage changes

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[DominiqueMakowski]

lol that was tedious!