Doc: minor changes to index.qdoc

index.qdoc of QtSerialPort

Task-number: QTBUG-32173
Change-Id: I0cc4a3fd0b469d344da952dfd1042e431e043a4b
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Martin Smith <martin.smith@digia.com>