vg: fix typos in Canvas.SetColor godoc strings

this was spotted during #258 :

fix doc on vg.Canvas.SetColor to read:

// SetColor sets the current drawing color.
// Note that fill color and stroke color are
// the same, so if you want different fill
// and stroke colors then you must set a color,
// draw fills, set a new color and then draw lines.
//
// The initial color is black.  If SetColor is
// called with a nil color then black is used.
SetColor(c color.Color)