Fix misleading comment in "thread" sample

Fixes a comment that is a bit confusing/misleading,

See wxWidgets#24525.

(cherry picked from commit a771ac6)

Author: Blake-Madden

samples/thread/thread.cpp

@@ -827,7 +827,7 @@ MyImageDialog::MyImageDialog(wxFrame *parent)
 
     CentreOnScreen();
 
-    // NOTE: no need to lock m_csBmp until the thread isn't started:
+    // NOTE: no need to lock m_csBmp until the thread has started:
 
     // create the bitmap
     if (!m_bmp.Create(GUITHREAD_BMP_SIZE,GUITHREAD_BMP_SIZE) || !m_bmp.IsOk())