Update string to account for new changes (#285)

Author: kythant

UpdateVersions.ps1

@@ -12,8 +12,8 @@ Get-ChildItem -Recurse packages.config -Path $PSScriptRoot | foreach-object {
 }
 
 Get-ChildItem -Recurse *.vcxproj -Path $PSScriptRoot | foreach-object {
-    $newVersionString = 'packages\Microsoft.WindowsAppSDK.' + $WinAppSDKVersion + '\'
-    $oldVersionString = 'packages\\Microsoft.WindowsAppSDK.[-.0-9a-zA-Z]*\\'
+    $newVersionString = '$(NugetPackageDirectory)\Microsoft.WindowsAppSDK.' + $WinAppSDKVersion + '\'
+    $oldVersionString = '$(NugetPackageDirectory)\Microsoft.WindowsAppSDK.[-.0-9a-zA-Z]*\\'
     $content = Get-Content $_.FullName -Raw
     $content = $content -replace $oldVersionString, $newVersionString
     Set-Content -Path $_.FullName -Value $content