fix datetime index in data_as_df method

[danielkrauss2]

(utc datetime was not localized to utc)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 43.87%. Comparing base (5b2eab0) to head (23c229a).
Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
src/biopsykit/io/psg.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   43.86%   43.87%   +0.01%     
==========================================
  Files          91       91              
  Lines        8057     8055       -2     
  Branches     1556     1555       -1     
==========================================
  Hits         3534     3534              
+ Misses       4460     4458       -2     
  Partials       63       63              

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