docs: fix data-model custom tag example and formatter name

[dajiaohuang]

Fix the Data Models guide's custom-tag example and formatter name

Key Changes:

• Add the missing colon to the custom-tag class example. As written, the documented snippet fails Python compilation with SyntaxError: expected ':'.
• Correct the instructed formatter method from the nonexistent .to_prety_xml() to Model.to_pretty_xml(), which is already used by the adjacent example and exists in rigging/model.py.

Added:

• None.

Changed:

• docs/topics/data-models.mdx (two lines).

Removed:

• None.

Verification performed on base 3461e81e799aff9040fe0d0671196b78667a7589:

• The original documented custom-tag snippet reproduces the SyntaxError; the corrected snippet compiles (compiled only, not executed).
• AST inspection of rigging/model.py confirms to_pretty_xml exists and to_prety_xml does not.
• Final diff review confirms exactly these two lines changed; selected rigging/ modules and tests compile syntax-only without importing the package.

Not run, and disclosed rather than claimed: the full Ruff/Mypy/pytest CI matrix, pre-commit hooks, and the always-run docs generator. This environment lacks the complete Poetry dependency closure (poetry install --all-extras) needed for those checks, and the isolated codespell module is also unavailable. This is a documentation-only change with no runtime source changes.

No review requested.