Update WebGLProgram.html

[Mugen87]

Adresses #10522

uv2 is not defined in WebGLProgram but in a shader chunk:

three.js/src/renderers/shaders/ShaderChunk/uv2_pars_vertex.glsl

Lines 1 to 6 in faa51af

	#if defined( USE_LIGHTMAP ) || defined( USE_AOMAP )
	attribute vec2 uv2;
	varying vec2 vUv2;
	#endif

[mrdoob]

Thanks!