Avoid assert on text sample startup under MSW

Use wxTE_RICH2 for a control that we call EnableProofCheck() on, as
using this function without this style asserts in wxMSW.

This should have been done in 5ba2461fd1 (Switch to using GSpell for
wxTextCtrl proof checker in wxGTK, 2021-09-08).

samples/text/text.cpp

@@ -1116,7 +1116,7 @@ MyPanel::MyPanel( wxFrame *frame, int x, int y, int w, int h )
 
     m_text = new MyTextCtrl( this, wxID_ANY, "Single line.",
                              wxDefaultPosition, wxDefaultSize,
-                             wxTE_PROCESS_ENTER);
+                             wxTE_PROCESS_ENTER | wxTE_RICH2);
     m_text->SetForegroundColour(*wxBLUE);
     m_text->SetBackgroundColour(*wxLIGHT_GREY);
     (*m_text) << " Appended.";