Fixed the url for appium-adb submodule.

Fix for this error:
```
paymand@payman:~/src/appium$ git submodule update --recursive
Cloning into 'submodules/appium-adb'...
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
Clone of 'git@github.com:appium/appium-adb.git' into submodule path 'submodules/appium-adb' failed
```