android readme: update "access datadir on Android from desktop"

was getting `cp: /sdcard/some_path/my_wallet: Operation not permitted`
adb no longer has permissions to write to the sdcard

New command allows dumping the file directly to local pc via usb.

related:
https://stackoverflow.com/q/72714568
https://stackoverflow.com/q/18471780

contrib/android/Readme.md

@@ -114,7 +114,8 @@ of Android does not let you access the internal storage of an app without root.
 ```
 $ adb shell
 $ run-as org.electrum.electrum ls /data/data/org.electrum.electrum/files/data
-$ run-as org.electrum.electrum cp /data/data/org.electrum.electrum/files/data/wallets/my_wallet /sdcard/some_path/my_wallet
+$ exit  # to exit adb
+$ adb exec-out run-as org.electrum.electrum cat /data/data/org.electrum.electrum/files/data/wallets/my_wallet > my_wallet
 ```
 
 Or use Android Studio: "Device File Explorer", which can download/upload data directly from device (via adb).