Update branch reference in README

Changed the branch name from 'main' to 'master' in the README to accurately reflect the current development branch for v6.

Author: mCodex

README.md

@@ -13,7 +13,7 @@ Modern secure storage for React Native, powered by Nitro Modules. Version 6 ship
 > Version 6 drops Windows support. The module now targets iOS and Android only.
 
 > [!IMPORTANT]
-> This README tracks the in-progress v6 work on `main`. For the stable legacy release, switch to the `v5.x` branch.
+> This README tracks the in-progress v6 work on `master`. For the stable legacy release, switch to the `v5.x` branch.
 
 ## Table of contents