Correct instructions for HTTPS and PowerShell (#5394)

Current instructions for running using HTTPS in a PowerShell prompt are incorrect and do not work on Windows 10 v1809

packages/react-scripts/template/README.md

@@ -1242,7 +1242,7 @@ set HTTPS=true&&npm start
 #### Windows (Powershell)
 
 ```Powershell
-($env:HTTPS = $true) -and (npm start)
+($env:HTTPS = "true") -and (npm start)
 ```
 
 #### Linux, macOS (Bash)