docs(BigTable): Fixing markdown mistakes in samples README #93933

Job	Run time
CI (ubuntu-latest, 3.1, --test)	59s
CI (ubuntu-latest, 3.4, --test)	1m 7s
CI (ubuntu-latest, 3.3, --test)	1m 9s
CI (windows-latest, 3.4, --test)	2m 51s
CI (ubuntu-latest, 3.4, --rubocop-toplevel --rubocop --build --yard --linkinator)	1m 8s
CI (macos-latest, 3.4, --test)	40s
CI (ubuntu-latest, 3.2, --test)	1m 6s
	9m 0s

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