[riot-notifications] [RIOT-OS/RIOT] examples/suit_update: update IETF draft reference in the README (#15418)

[Emmanuel Baccelli]

### Contribution description

the readme pointed to an outdated version of the IETF draft specification for the SUIT manifest.
Since the code now complied with version -09 of draft, this PR fixes the reference and the URL in the readme of the example.


### Testing procedure

look at the readme ;)


### Issues/PRs references

not applicable.

You can view, comment on, or merge this pull request online at:

  https://github.com/RIOT-OS/RIOT/pull/15418

-- Commit Summary --

  * examples/suit_update: update IETF draft reference

-- File Changes --

    M examples/suit_update/README.md (2)

-- Patch Links --

https://github.com/RIOT-OS/RIOT/pull/15418.patch
https://github.com/RIOT-OS/RIOT/pull/15418.diff

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