Add missing THEN keyword to if-statement. (dotnet#4876)

The example for `GetDirectoryName()` had a method with an if-statement that was missing the `Then` keyword making it invalid syntax (the Visual Studio editor usually fixes this automatically but for some reason it was missed here).

Author: ericmutta

samples/snippets/visualbasic/VS_Snippets_CLR_System/system.IO.Path Members/VB/pathmembers.vb

@@ -55,7 +55,7 @@ Public Class PathSnippets
             Console.WriteLine("GetDirectoryName('{0}') returns '{1}'", _
                 filepath, directoryName)
             filepath = directoryName
-            If i = 1
+            If i = 1 Then
                filepath = directoryName + "\"  ' this will preserve the previous path
             End If
             i = i + 1
@@ -333,4 +333,4 @@ Public Class PathSnippets
         pathnameSnippets.HasExtension()
         pathnameSnippets.IsPathRooted()
     End Sub
-End Class
+End Class