Fix MoveTo docs regarding origin for offsets

[derat]

WebElement.MoveTo's documentation stated that the supplied offsets are interpreted relative to the element's center, but this seems to be incorrect. Since remoteWE's implementation always passes "xoffset" and "yoffset" parameters, the offsets are interpreted relative to the element's top-left corner.

---

The spec states that the mouse is moved to the center of the element if no offsets are supplied, but that offsets are interpreted relative to the top-left corner of the element.

Here's the implementation, which unconditionally passes offsets:

selenium/remote.go

Lines 1415 to 1421 in e9100b7

	func (elem *remoteWE) MoveTo(xOffset, yOffset int) error {
	return elem.parent.voidCommand("/session/%s/moveto", map[string]interface{}{
	"element": elem.id,
	"xoffset": xOffset,
	"yoffset": yOffset,
	})
	}

See also #190.