[Fix] build docs warning

[danibene]

Description

This PR aims to fix the failed documentation build check: https://github.com/neuropsychology/NeuroKit/actions/runs/8094952153/job/22120703414

Proposed Changes

I changed the emg_activation() function to index a DataFrame with .loc

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 targeted 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

Attention: Patch coverage is 35.29412% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 54.59%. Comparing base (56aba7f) to head (6f0f0a2).
Report is 3 commits behind head on dev.

Files	Patch %	Lines
neurokit2/complexity/utils_complexity_symbolize.py	0.00%	9 Missing ⚠️
neurokit2/emg/emg_activation.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #968      +/-   ##
==========================================
- Coverage   54.59%   54.59%   -0.01%     
==========================================
  Files         304      304              
  Lines       14325    14326       +1     
==========================================
  Hits         7821     7821              
- Misses       6504     6505       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[DominiqueMakowski]

merge whenever you want (I've been away from NK for some time but will return soon) - we should probably aim at releasing the dev in a close future :)

[danibene]

merge whenever you want (I've been away from NK for some time but will return soon) - we should probably aim at releasing the dev in a close future :)

Sounds good! Hope you're well