docs: Fix missing reference warning in ModuleSystem.md

This commit addresses the following warning introduced
in 33fbaba2 (Updating doc)

/path/to/VTK/Documentation/docs/api/cmake/ModuleSystem.md:1005:
WARNING: local id not found in doc 'advanced/spdx_and_sbom': 'frequently_asked_questions' [myst.xref_missing]