bluetooth: README.md fixes for gitiles rendering.
bluetooth: README.md fixes for gitiles rendering.

Turns out gitiles doesn't support markdown tables. Changed to just be
pre-formated code block.

Horizontal rules aren't supported either, but aren't needed as there
is ample spacing.

R=ortuno@chromium.org

Review URL: https://codereview.chromium.org/1644453003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#371985}

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