Update MSI install docs for Powershell

[kilfoyle]

From Bill:

For this MSI doc https://www.elastic.co/guide/en/fleet/current/install-agent-msi.html

If the user runs the command via Powershell we need them to use a slightly different command:

./elastic-agent-8.15.0-windows-x86_64.msi --% INSTALLARGS="--url=url --enrollment-token=token=="

with the executable targeted with ./ and --% after the MSI to prevent Powershell from adding quotes around INSTALLARGS due to the space between url and enrollment token