Update BUILDING.md

[AaronMbuzi]

Updated BUILDING.md titles so that there is consistency throughout the documentation.

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?
• If submitting code, have you built your formula locally prior to submission with gradle check?
• If submitting code, is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed.
• If submitting code, have you checked that your submission is for an OS and architecture that we support?
• If you are submitting this code for a class then read our policy for that.

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💚 CLA has been signed

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[AaronMbuzi]

❌ Author of the following commits did not sign a Contributor Agreement: b699d95

Please, read and sign the above mentioned agreement if you want to contribute to this project

It has been singed

[elasticsearchmachine]

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