Update developer instructions in `install_trustchain_mobile.md`

The instructions in `install_trustchain_mobile.md` are out of date. The config parameters in `flutter-config.json` should be:
 - `trustchainEndpoint`
 - `rootEventDate`
 - `confirmationCode`
 - `rootDid`
 - `rootTxid`
 - `rootBlockHeight`
 - `rootEventTime`