Minor: Fix some strings

"Todo" -> "To-do" for consistency reasons

plugins/messageviewerplugins/createtodoplugin/todoedit.cpp

@@ -59,7 +59,7 @@ TodoEdit::TodoEdit(QWidget *parent)
     hbox->setSpacing(2);
     vbox->addLayout(hbox);
 
-    auto lab = new QLabel(i18n("Todo:"), this);
+    auto lab = new QLabel(i18n("To-do:"), this);
     hbox->addWidget(lab);
 
     mNoteEdit->setClearButtonEnabled(true);

plugins/messageviewerplugins/createtodoplugin/viewerplugincreatetodointerface.cpp

@@ -65,7 +65,7 @@ ViewerPluginInterface::SpecificFeatureTypes ViewerPluginCreateTodoInterface::fea
 void ViewerPluginCreateTodoInterface::createAction(KActionCollection *ac)
 {
     if (ac) {
-        auto act = new QAction(QIcon::fromTheme(QStringLiteral("task-new")), i18n("Create Todo"), this);
+        auto act = new QAction(QIcon::fromTheme(QStringLiteral("task-new")), i18n("Create To-do"), this);
         act->setIconText(i18n("Create To-do"));
         addHelpTextAction(act, i18n("Allows you to create a calendar to-do or reminder from this message"));
         act->setWhatsThis(