Updating command for authentication on windows

Author: devshah-LT24

README.md

@@ -66,7 +66,8 @@ Set LambdaTest `Username` and `Access Key` in environment variables.
   ```
   * For **Windows**:
   ```bash
-  set LT_USERNAME="YOUR_USERNAME" set LT_ACCESS_KEY="YOUR ACCESS KEY"
+  $env:LT_USERNAME='YOUR_USERNAME'
+  $env:LT_ACCESS_KEY='YOUR ACCESS KEY'
   ```
 
 ## Run Your First Test