Fix promote_rc.sh: Update Homebrew formula path for apache-geode

[JinwooHwang]

In December 2021, Homebrew reorganized the homebrew-core repository structure to improve performance as the number of formulas grew. The flat Formula/ directory was split into alphabetically organized subdirectories (Formula/a/, Formula/b/, etc.).

Since apache-geode starts with 'a', the formula file moved from:
Formula/apache-geode.rb
to:
Formula/a/apache-geode.rb

This change updates promote_rc.sh to cd into Formula/a/ instead of Formula/, which has been the correct structure for over 3 years.

Without this fix, the script fails with 'No such file or directory' when attempting to update the apache-geode formula during the release promotion process.

Reference: Homebrew/homebrew-core#91469

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