Release Post: Clarify social media image instructions

Why is this change being made?

In the 15.5 release post retro, the release post manager was:

... very confused by the social media cover image directions.

This MR clarifies the steps required to update the two places where the image path should be added to the release post intro.

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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