🐛 fixes wrong variable used in AboutStringBuilder (#380)

Author: Maamue

PSKoans/Koans/Constructs and Patterns/AboutStringBuilder.Koans.ps1

@@ -157,7 +157,7 @@ Describe 'System.Text.StringBuilder' {
             Get-Member |
             Where-Object MemberType -eq 'Property'
 
-            $ExpectedCount = $Properties |
+            $ExpectedPropertyCount = $Properties |
             Measure-Object |
             Select-Object -ExpandProperty Count