[Docs] typos

[danibene]

Proposed Changes

I fixed some typos in PULL_REQUEST_TEMPLATE.md, entropy_shannon(), and hrv_nonlinear().

Edit: I have also attempted to address a FutureWarning raised in the docs check by ecg_delineate

Edit x2: This attempt failed (see #897) so nevermind, let's just merge the typo changes

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 has no change and project coverage change: -0.04% ⚠️

Comparison is base (60884fb) 55.22% compared to head (619c8d6) 55.19%.
Report is 2 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #896      +/-   ##
==========================================
- Coverage   55.22%   55.19%   -0.04%     
==========================================
  Files         302      302              
  Lines       14141    14142       +1     
==========================================
- Hits         7810     7805       -5     
- Misses       6331     6337       +6     

Files Changed	Coverage Δ
neurokit2/complexity/entropy_shannon.py	83.33% <ø> (ø)
neurokit2/hrv/hrv_nonlinear.py	66.66% <ø> (ø)

... and 2 files with indirect coverage changes

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