fix typo "pointsize"

[amaya382]

When I use a pointSize option, failed to generate output with no error.

output(PNG("./", "a"), xyChart(Seq(1.0d, 2.0d, 3.0d) -> Y(Seq(2.0d, 3.0d, 4.0d), style = XYPlotStyle.Points), pointSize = Some(3.0d)))

And then, I execute gnuplot manually by gnuplot a.gpl, I got following errors.

set pointSize 3.000000
    ^
"a.gpl", line 2: unrecognized option - see 'help set'.