Update link to Mustache spec and not JS implementation (#26950)
Update link to Mustache spec and not JS implementation (#26950)

For context see here: https://docs.google.com/document/d/1lsG9svgClzfknEwWD8mMc7kf91N8RJhGrssNPc0lO9E/edit#bookmark=id.1xum78ltl3cu
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