Fix Helm chart release guide

[kaxil]

Before:

❯ for f in ../../../airflow-dev/helm-chart/$RC/*; do svn cp $f ${$(basename $f)/rc?/}; done

A         airflow-1.1.0.tgz
A         airflow-1.1.0.tgz.asc
A         airflow-1.1.0.tgz.prov
A         airflow-1.1.0.tgz.sha512
A         airflow-chart-1.1.0-sou.tar.gz
A         airflow-chart-1.1.0-sou.tar.gz.asc
A         airflow-chart-1.1.0-sou.tar.gz.sha512
A         index.yaml

After:

❯ for f in ../../../airflow-dev/helm-chart/$RC/*; do svn cp $f ${$(basename $f)/}; done

A         airflow-1.1.0.tgz
A         airflow-1.1.0.tgz.asc
A         airflow-1.1.0.tgz.prov
A         airflow-1.1.0.tgz.sha512
A         airflow-chart-1.1.0-source.tar.gz
A         airflow-chart-1.1.0-source.tar.gz.asc
A         airflow-chart-1.1.0-source.tar.gz.sha512

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^ Add meaningful description above

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In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.

[github-actions]

The PR is likely ready to be merged. No tests are needed as no important environment files, nor python files were modified by it. However, committers might decide that full test matrix is needed and add the 'full tests needed' label. Then you should rebase it to the latest main or amend the last commit of the PR, and push it with --force-with-lease.