Update pathcombine.cs example comment (#4441)

Comment for the last example incorrectly stated the output of the test routine; it showed '' instead of 'null'

Author: BoldAsLove

samples/snippets/csharp/VS_Snippets_CLR/pathcombine/CS/pathcombine.cs

@@ -57,7 +57,7 @@ private static void CombinePaths(string p1, string p2) {
 // When you combine '' and 'subdir\file.txt', the result is:
 // 'subdir\file.txt'
 //
-// You cannot combine '' and 'subdir\file.txt' because:
+// You cannot combine 'null' and 'subdir\file.txt' because:
 // Value cannot be null.
 // Parameter name: path1
-//</snippet1>
+//</snippet1>