Upgrades reveal mermaid plugin to 2.1.0

[kriberg]

Upgrades to reveal.js-mermaid-plugin 2.1.0 which has mermaid.js 10.6.1.

Had some issues with displaying this diagram:

flowchart LR
	Domain -- has many --> System
	System -- has many --> Component
	Component -- provides --> API
	API -. used by .-> Component
		

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The arrow heads were not visible. Upgrading the plugin to 2.1.0 fixed the issue. Suppose it was a bug in the 9.x build of mermaid.