Fix misspellings in comments and documentation

No real changes.

Closes https://github.com/wxWidgets/wxWidgets/pull/870

src/gtk/radiobox.cpp

@@ -290,7 +290,7 @@ bool wxRadioBox::Create( wxWindow *parent, wxWindowID id, const wxString& title,
                          // So far we assumed that label doesn't contain mnemonic
                          // and therefore single underscore characters were not
                          // replaced by two underscores. Now we have to double
-                         // all exisiting underscore characters.
+                         // all existing underscore characters.
                          label.Replace(wxS("_"), wxS("__"));
                          label += wxS('_');
                      }