dev README source release section contains stale Airflow 2.0 and provider path references

[Tomeshwari-02]

What happened

The source release documentation in dev/README.md appears to contain several stale references that no longer match the current repository layout and release state on main.

Observed on main at c22cc6d:

• dev/README.md:45 says the source package builds a "fully-functional Apache Airflow 2.0 package". The current development branch is no longer describing an Airflow 2.0 source package.
• dev/README.md:56 says "We also plan to release the official Helm Chart sources...", but the repository already contains the Helm chart under chart/.
• dev/README.md:89 points readers to provider package files in airflow/providers/<PROVIDER>, while providers now live under top-level directories such as providers/standard, providers/amazon, and providers/http.
• dev/README.md:92 has official_source with an underscore.
• dev/README.md:99 has a missing space in "apache-airflow-providers"separately.

These are small documentation issues, but they are in release/development documentation and can confuse contributors or release-adjacent readers trying to understand the current source tree.

What you think should happen instead

The dev/README.md source release section should be refreshed so that it:

• Describes the current Airflow source package without referring specifically to Airflow 2.0.
• Reflects that the Helm chart is already part of the repository.
• Points provider readers to the current providers/<provider>/... layout.
• Fixes the minor wording/spacing typos around official source and apache-airflow-providers.

How to reproduce

1. Open dev/README.md on the current main branch.
2. Review the "Apache Airflow Package" and "Provider distributions" sections.
3. Compare the documented provider path with the current top-level providers/ tree.

[boring-cyborg]

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