Fix comment typos in sources

No real changes.

Closes https://github.com/wxWidgets/wxWidgets/pull/2541

include/wx/propgrid/propgridiface.h

@@ -324,7 +324,7 @@ public:
     const wxPGAttributeStorage& GetPropertyAttributes( wxPGPropArg id ) const
     {
         // If 'id' refers to invalid property, then we will return dummy
-        // attributes (i.e. root property's attributes, which contents should
+        // attributes (i.e. root property's attributes, which contents
         // should always be empty and of no consequence).
         wxPG_PROP_ARG_CALL_PROLOG_RETVAL(m_pState->DoGetRoot()->GetAttributes())
         return p->GetAttributes();