Fix a grammar issue of the open method documentation
Fix a grammar issue of the open method documentation

Change-Id: I158a5a66af5c07d4454a559d4e5d6b7a666f43a7
Reviewed-by: Sergey Belyashov <8871b4c2d635fca691211507eadbac3d0bc2695a@gmail.com>
Reviewed-by: Denis Shienkov <6a9c747bd2ba446682324cf340d3b86f30cf4d55@gmail.com>

kobolabs/qtserialport,RoadWorksSoftware/QtSerialPort-OS2,kobolabs/qtserialport,RoadWorksSoftware/QtSerialPort-OS2