docs: fix the diffpy.utils.transforms.d_to_tth example

[ycexiao]

What problem does this PR address

Closes #352

Remove a d value from the documentation that will raise ValueError.

[github-actions]

Warning! No news item is found for this PR. If this is a user-facing
change/feature/fix,
please add a news item by copying the format from news/TEMPLATE.rst.
For best practices, please visit
https://scikit-package.github.io/scikit-package/frequently-asked-questions.html#billinge-group-standards.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (44328a2) to head (3ea591b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #357   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          520       520           
=========================================
  Hits           520       520           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ycexiao]

@sbillinge, it's ready for review. The news file has already been added in #354. These two PR address the same problem (documentation examples), so I didn't create a new file for this one.