samples: hello: fix auth cert symbol format

[szczys]

An extra set of single quotes in the Kconfig symbol for certificate authentication has been removed to prevent a build warning that is causing the symbols to be ignored:

warning: malformed string literal in assignment to GOLIOTH_SYSTEM_CLIENT_CRT_PATH

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 06905d4):

https://golioth-zephyr-sdk-doxygen-dev--pr388-szczys-cert-symb-f0232o36.web.app

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