Update Sharepoint site logo image using powershell

[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.Sharepoint")
# Provide Site URL
$Url="https://mySharepointSite.Company.com/sites/demo/default.aspx"
$site = new-object Microsoft.SharePoint.SPSite($Url)
$web=$site.OpenWeb();
$web.SiteLogoUrl = "/demo/shared documents/logo.gif"
$web.Update()
$web.Close()
$site.Dispose()