Shaders: Fix comment line (#7668)

Fixed comment as finalColorBlend() does not exist in the code base.

client/shaders/nodes_shader/opengl_vertex.glsl

@@ -135,7 +135,7 @@ float disp_z;
	color.a = 1;

	// Emphase blue a bit in darker places
	// See C++ implementation in mapblock_mesh.cpp finalColorBlend()
	// See C++ implementation in mapblock_mesh.cpp final_color_blend()
	float brightness = (color.r + color.g + color.b) / 3;
	color.b += max(0.0, 0.021 - abs(0.2 * brightness - 0.021) +
		0.07 * brightness);