fix(docs): specify python version to 3.10

The docs say 3 is valid, but it is not. Jammy uses 3.10 so pin it to
that.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I73530750065294eb511d88318ba86a6c50c8aa7d