RFR: 7794: Update the link to the adoptium download site

Fri May 13 10:53:23 UTC 2022

On Wed, 13 Apr 2022 23:04:36 GMT, Bernd <duke at openjdk.java.net> wrote:

> The AdoptOpenJDK no longer has the latest builds.
> Also use https for Links.

Changes requested by hirt (Lead).

Marked as reviewed by hirt (Lead).

README.md line 18:

> 16: * Downloadable Eclipse update site archive
> 17: 
> 18: [https://adoptium.net/de/jmc](https://adoptium.net/de/jmc)

I think the URL should likely be:
https://adoptium.net/jmc/

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PR: https://git.openjdk.java.net/jmc/pull/397