samples: wifi: ble_coex: Readme update

[muraliThokala]

Update readme to suit the latest documentation style.

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Test specification

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CI/Jenkins/integration

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CI Information

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Build number: 34

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Enabled	Name	Description
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	ci-all-test	Run all of ci, no test spec filtering will be done
	ci-force-downstream	Force execution of downstream even if twister fails
	ci-run-twister	Force run twister
	ci-run-zephyr-twister	Force run zephyr twister

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Test Spec & Results: ✅ Success; ❌ Failure; 🟠 Queued; 🟡 Progress; ◻️ Skipped; ⚠️ Quarantine

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