QtIFW

Fix readFile documentation

readFile method returns string and not boolean. Fix it to documentation.

Change-Id: I8454b423a01e93c747087de662e6795a72db26bd
Reviewed-by: Karim Pinter <karim.pinter@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Iikka Eklund <iikka.eklund@qt.io>