gh-35287: add missing colon in `finite_drinfeld_module.py`
<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes #1234" use "Introduce new method to
calculate 1+1"
-->
### 📚 Description
There is a missing colon which makes the doc displaying incorrectly.
<!-- Describe your changes here in detail -->
<!-- Why is this change required? What problem does it solve? -->
<!-- If it resolves an open issue, please link to the issue here. For
example "Closes #1337" -->
### 📝 Checklist
<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
- [ ] I have made sure that the title is self-explanatory and the
description concisely explains the PR.
- [ ] I have linked an issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### ⌛ Dependencies
<!-- List all open pull requests that this PR logically depends on -->
<!--
- #xyz: short description why this is a dependency
- #abc: ...
-->
@kryzar @ymusleh @xcaruso
URL: https://github.com/sagemath/sage/pull/35287
Reported by: David Ayotte
Reviewer(s): Antoine Leudière, David Coudert
