docs: use syntax highlighting

[yeikel]

use syntax highlighting to improve the format of the readme

Preview : https://github.com/yeikel/JSON-java/blob/patch-1/README.md

[stleary]

What problem does this code solve?
Updates the readme markdown

Does the code still compile with Java6?
N/A

Risks
Low

Changes to the API?
N/A

Will this require a new release?
No

Should the documentation be updated?
This updates the docs

Does it break the unit tests?
N/A

Was any code refactored in this commit?
N/A

Review status
APPROVED

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