docs: use brew official formula (#3733)

README.md

@@ -28,7 +28,7 @@ curl -fsSL https://opencode.ai/install | bash
 npm i -g opencode-ai@latest        # or bun/pnpm/yarn
 scoop bucket add extras; scoop install extras/opencode  # Windows
 choco install opencode             # Windows
-brew install sst/tap/opencode      # macOS and Linux
+brew install opencode      # macOS and Linux
 paru -S opencode-bin               # Arch Linux
 ```