Doc: Fixed small typo in graphics topic page.

Change-Id: I20f773ee9c37a52250a4aeb464047a85e2de0a06
Task-number: QTWEBSITE-563
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>