docs: fix successful spelling in README

[yosinn1-blip]

Changes:

• succesful -> successful in hbt/src/ringbuffer/README.rst (1 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.

[meta-cla]

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