QString: Fix snippet explaining "non-spaced numbered place markers"

The snippet tries to explain the advantage of

 str.arg("Hello", QString::number(20), QString::number(50));

over

 str.arg("Hello").arg(20).arg(50);

which only makes sense, if str contains the same formating sequence,
namely "%1%3%2". That also matches the belonging comment.

Amends 8481500f63

Task-number: QTBUG-44044
Change-Id: Ic2595107bc599c6d244ebf88184a5cc5569ed4f1
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/doc/snippets/qstring/main.cpp

@@ -291,7 +291,7 @@ void Widget::argFunction()
     //! [97]
 
     //! [98]
-    str = "%1%2%3";
+    str = "%1%3%2";
     str.arg("Hello", QString::number(20), QString::number(50)); // returns "Hello5020"
     //! [98]