docs: displaying environment variable as code snippet

[ghost]

Added the markdown syntax to the singular environment variable in the README.md not displayed as a code snippet to be displayed as one, increasing readability.

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[ghost]

Same change as https://github.com/aws/aws-lambda-runtime-interface-emulator/pull/57 with a more specific commit message, both PRs can be closed after either is merged.

[ghost]

Closing in favour of https://github.com/aws/aws-lambda-runtime-interface-emulator/pull/57

[valerena]

Merged in #57