Fix convertTrials tutorial (#515)

[ehennestad]

Fix #515

Motivation

Fix outdated tutorial

How to test the behavior?

Run tutorial

The tutorial should run without errors and the exported file can be read with matnwb and pynwb v2.8.1

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.63%. Comparing base (1b7b657) to head (c81a78b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #594      +/-   ##
==========================================
+ Coverage   90.60%   90.63%   +0.03%     
==========================================
  Files         106      106              
  Lines        4671     4687      +16     
==========================================
+ Hits         4232     4248      +16     
  Misses        439      439              

Flag	Coverage Δ
	90.63% <ø> (?)

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