docs: fix separated spelling in docs

[yosinn1-blip]

Changes:

• seperated -> separated in doc/source/overview_ring_format.rst (2 occurrence(s), preserving capitalization where applicable)

Verification:

• Applied an exact spelling-only replacement against the current upstream base branch.
• Checked the repository is not archived or a fork.
• Checked open PR titles and my open PRs before opening this PR.

[openstackadmin]

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