Remove unnecessary flag when running JS server (#25517)

Summary:
This removes the `--projectRoot` flag in `launchPackager.bat` as it was removed,  fixing [this bundler not opening issue on cli repo](https://github.com/react-native-community/cli/issues/484) when `yarn react-native run-android` is run.

This is Windows related only so should be only tested on Windows.

## Changelog

[Internal] [Fixed] - Fixed Metro Bundler not opening when running `yarn react-native run-android`
Pull Request resolved: https://github.com/facebook/react-native/pull/25517

Test Plan:
1. Create a new repo using `react-native init`
2. Edit the `launchPackager.bat` file
3. Run `yarn react-native run-android`

Differential Revision: D16162108

Pulled By: cpojer

fbshipit-source-id: c12d9853ad49f00d56b9a8254a5b2c40a358cb2e

scripts/launchPackager.bat

@@ -6,6 +6,7 @@
 @echo off
 title Metro Bundler
 call .packager.bat
-node "%~dp0..\cli.js" --projectRoot ../../../ start
+cd ../../../
+node "%~dp0..\cli.js" start
 pause
 exit