pacific: doc/start: fix "Planet Ceph" link

[zdover23]

Fix a link to Planet Ceph on the doc/start/get-involved.rst page.

Reported 2023 Apr 21, here:
https://pad.ceph.com/p/Report_Documentation_Bugs

Signed-off-by: Zac Dover zac.dover@proton.me
(cherry picked from commit 67ebc20)

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