Fix broken internal links in README

[carbeer]

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Resolves #

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[knsv]

For me these liks already work like for instance:
https://mermaid-js.github.io/mermaid/#/flowchart?id=flowcharts-basic-syntax

In what context are they broken?

[carbeer]

The issue arises when using the README on Github directly.

[knsv]

@carbeer Thanks Carolin! We really want it to work on Github as well!