[docs] Fix return value of QStringIterator::next()

It's char32_t these days, not uint.

Task-number: QTBUG-103531
Pick-to: 6.4 6.3 6.2
Change-Id: Iaa03f97d0d1266a6763eb858edb45ae0f2a4729d
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/doc/snippets/code/src_corelib_text_qstringiterator.cpp

@@ -16,7 +16,7 @@ QStringIterator i(string); // implicitly converted to QStringView
 
 //! [1]
 while (i.hasNext())
-    uint c = i.next();
+    char32_t c = i.next();
 //! [1]
 }