Doc: Updated the code snippets for the toLocalexxx functions

Using Date() as a regular function without the new operator
returns a string and not a date object.

Change-Id: I083bb62c0cb3041a053d43e3085c3d0d10423db6
Task-number: QTBUG-41712
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>

src/qml/doc/src/javascript/date.qdoc

@@ -153,7 +153,7 @@
     import QtQuick 2.0
 
     Text {
-        text: "The date is: " +  Date().toLocaleString(Qt.locale("de_DE"))
+        text: "The date is: " + new Date().toLocaleString(Qt.locale("de_DE"))
     }
     \endcode
 */
@@ -175,7 +175,7 @@
     import QtQuick 2.0
 
     Text {
-        text: "The date is: " +  Date().toLocaleDateString(Qt.locale("de_DE"))
+        text: "The date is: " + new Date().toLocaleDateString(Qt.locale("de_DE"))
     }
     \endcode
 */
@@ -197,7 +197,7 @@
     import QtQuick 2.0
 
     Text {
-        text: "The date is: " +  Date().toLocaleTimeString(Qt.locale("de_DE"))
+        text: "The date is: " + new Date().toLocaleTimeString(Qt.locale("de_DE"))
     }
     \endcode
 */