The article says that "If the plane makes an angle of with the x-axis, then the normal vector makes an angle of " (adding the for the right angle of the normal vector). The x-coordinate of such a vector is , the y-coordinate is 0 because the plane contains the y-axis, and the z coordinate is . That would give vector . They have simplified slightly by multiplying by 2 which changes the length but not the direction of the vector.