Fix powershell instructions

Author: twangboy

README.rst

@@ -374,7 +374,7 @@ Using ``PowerShell`` to install latest stable version:
 
   [System.Net.ServicePointManager]::SecurityProtocol = [System.Net.SecurityProtocolType]'Tls12'
   Invoke-WebRequest -Uri https://github.com/saltstack/salt-bootstrap/releases/latest/download/bootstrap-salt.ps1 -OutFile "$env:TEMP\bootstrap-salt.ps1"
-  Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser & "$env:TEMP\bootstrap-salt.ps1"
+  Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser -Force; & "$env:TEMP\bootstrap-salt.ps1"
 
 Display information about the install script parameters: