fix version semantics for 0.6.dev2

[jo-mueller]

In the previously submitted proposal of RFC5, the associated version was named 0.6dev2 in contrast to the previously used 0.6.dev2. This led to problems elsewhere, so this is reverting it back. It has also been changed accordingly in these PRs:

• https://github.com/ome/ngff-spec/pull/17
• https://github.com/jo-mueller/ngff-rfc5-coordinate-transformation-examples/pull/14

This should make the versioning semantic consistent across all relevant places. The ome ngff validator has already been made robust to such changes here

Edit: typos

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[lubianat]

@jo-mueller with the ngff-spec infra work, does this pr still make sense?

[jo-mueller]

@lubianat hm...I think this can still go in. It's just to make the RFC docs consistent with the actual spec text.