[Issue-633] Fix the link to Platforms Definition white paper in Platform Engineering Maturity Model

[tiagodolphine]

Closes #633

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[lianmakesthings]

Fixed already in #625

[abangser]

Thanks for the contribution @tiagodolphine, you happened to stumble in in the middle of an ongoing fix where we were prioritising keeping the URLs stable from initial release which is why we solved in a slightly different way in #625. Let us know if there are any other issues with this fix in place. Thanks!