Update trails.rst

[jwd83]

Fixed a word in the documentation that doesn't match what the setting is now called in the current version. <!--
Please target the master branch in priority.
PRs can target other branches (e.g. 3.2, 3.5) if the same change was done in master, or is not relevant there.
PRs must not target stable, as that branch is updated manually.

The type of content accepted into the documentation is explained here:
https://docs.godotengine.org/en/latest/community/contributing/content_guidelines.html
-->

[skyace65]

Thanks! And congrats on your first merged PR!

[mhilbrunner]

Cherry-picked to master. For future PRs, please target the master branch first, we'll worry about cherry-picking to the release branches. It is also a good idea to have a commit message/PR title that explains what is actually changed :) Thanks for contributing!