Fix typo in model/view docs.

Task-number: QTWEBSITE-560

Change-Id: I2b7a6f28ac41bacbfd2603feb8b786c31d3769e3
Reviewed-by: Tero Kojo <tero.kojo@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>