docs: add -p to mkdir command

[westonplatter]

Summary & Motivation

Without -p, I experience this on a mac.

$ mkdir src/dagster_quickstart/defs/data && touch src/dagster_quickstart/defs/data/sample_data.csv

=> mkdir: src/dagster_quickstart/defs: No such file or directory

Test Plan

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[greptile-apps]

Greptile Summary

This PR fixes the quickstart documentation by adding the -p flag to mkdir, which ensures intermediate parent directories are created when they don't yet exist. Without it, the command fails on a fresh checkout where src/dagster_quickstart/defs doesn't exist, blocking users following the guide. The command is also split across two lines for readability.

Confidence Score: 5/5

Safe to merge — single-line docs fix with no code changes.

The change is a minimal, correct documentation fix. Adding -p to mkdir is the standard solution for this exact class of error and the PR description confirms the bug with a real failure output. No logic or code is changed.

No files require special attention.

Important Files Changed

Filename	Overview
docs/docs/getting-started/quickstart.md	Adds -p flag to mkdir so nested directories are created correctly, and splits the command across two lines for readability.

Sequence Diagram

sequenceDiagram
    participant User
    participant Shell

    Note over User,Shell: Before fix (no -p flag)
    User->>Shell: mkdir src/dagster_quickstart/defs/data
    Shell-->>User: Error: No such file or directory

    Note over User,Shell: After fix (with -p flag)
    User->>Shell: mkdir -p src/dagster_quickstart/defs/data
    Shell-->>User: Creates all intermediate dirs
    User->>Shell: touch src/dagster_quickstart/defs/data/sample_data.csv
    Shell-->>User: File created successfully

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