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Fix outdated tutorial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

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/NeurodataWithoutBorders/matnwb/pull/594/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/NeurodataWithoutBorders/matnwb/pull/594/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders) | `90.63% <ø> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders#carryforward-flags-in-the-pull-request-comment) to find out more.

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