chore: Update release cadence in README

The release cadence is now described as "roughly bi-weekly" instead of "weekly".

PiperOrigin-RevId: 803528472

Author: hangfei

README.md

@@ -64,7 +64,7 @@ You can install the latest stable version of ADK using `pip`:
 pip install google-adk
 ```
 
-The release cadence is weekly.
+The release cadence is roughly bi-weekly.
 
 This version is recommended for most users as it represents the most recent official release.