[Doc/WaterVapourDensity] Corrected a Quality Assurance issue

wenqingrequested to merge
wenqing/ogs:MPL_vapour_fixing into master

A quick fixing an issue of Documentation pages that are not referenced in the source code.

  1. Feature description was added to the changelog
  2. Tests covering your feature were added?
  3. Any new feature or behavior change was documented?

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