chore(Data/Ordmap): put examples in a code block (leanprover-community#38121)

This PR prepares for enabling Verso docstrings by replacing the four space indented code blocks in Ordmap/Ordnode.lean with backticks. Although four spaces are supported in some Markdown dialects, doc-gen4 and Verso do not support it.

Spotted while going through `linter.style.docStringVerso` warnings.

Co-authored-by: Anne C.A. Baanen <vierkantor@vierkantor.com>
Co-authored-by: pre-commit-ci-lite[bot] <117423508+pre-commit-ci-lite[bot]@users.noreply.github.com>

Author: Vierkantor