Fix grammar in Data Structures definition

[Nullkernel]

Motivation

• Correct a duplicated word and add the Oxford comma in the Data Structures definition to improve clarity and grammar.

Description

• Replace the fragment "It is is a way to organize, store and manage data..." with "It is a way to organize, store, and manage data..." in Notes/Day-1-intro.md while preserving the existing markdown structure and formatting.

Testing

• Verified the updated line and file contents with nl -ba Notes/Day-1-intro.md | sed -n '10,18p' and confirmed a single-line substitution in the file diff (checks passed).

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Codex Task