Update canvas_item_shader.rst

I tested it and it is upper left, not lower left.
This is the CanvasItem shader I used to test it:

shader_type canvas_item;

void fragment() {

	if(distance(FRAGCOORD.xy, vec2(40, 40)) < 30.0)
		COLOR = vec4(1,0,0,1);
	else
		COLOR = vec4(0,0,0,1);
}

(cherry picked from commit b71cec668abe3ba541a2712a2342bca9489b03f7)

tutorials/shaders/shader_reference/canvas_item_shader.rst

@@ -159,7 +159,7 @@ it to the ``NORMALMAP`` property. Godot will handle converting it for use in 2D
 | Built-in                                    | Description                                                   |
 +=============================================+===============================================================+
 | in vec4 **FRAGCOORD**                       | Coordinate of pixel center. In screen space. ``xy`` specifies |
-|                                             | position in window. Origin is lower-left.                     |
+|                                             | position in window. Origin is upper-left.                     |
 +---------------------------------------------+---------------------------------------------------------------+
 | in vec2 **SCREEN_PIXEL_SIZE**               | Size of individual pixels. Equal to inverse of resolution.    |
 +---------------------------------------------+---------------------------------------------------------------+