Doc: fix link to Qt Creator Manual
Doc: fix link to Qt Creator Manual

Replace the actual URL with a variable.

Change-Id: I5772d732fe3fdf2e5f5f12c415dfb49cfab8152e
Reviewed-by: Jerome Pasion <c10995d24f6b454503389e8d00d837aef038985d@digia.com>

kobolabs/qtdoc,kobolabs/qtdoc,kobolabs/qtdoc,kobolabs/qtdoc