Fix misleading type_from_binary instructions in file-sniffer

[frozar]

This PR fixes a misleading sentence in the File Sniffer concept exercise.

The type_from_binary/1 function receives a binary file and determines the media type from the file signature. The previous wording mentioned the extension not matching, but extensions are handled by type_from_extension/1 and verify/2.

This change clarifies that type_from_binary/1 returns nil when the file signature does not match the expected ones.

[github-actions]

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Automated comment created by PR Commenter 🤖.

[angelikatyborska]

Thank you, this makes sense!