doc: include attribute position for vertex attrib pointer

[CallumFerguson]

In the guide's WebGL example, vertex_attrib_pointer_with_i32 is used which takes an attribute location (indx). The attributes location position_attribute_location should be used instead of a hard coded 0.

You can see #2609 updated

context.enable_vertex_attrib_array(0);

to be

context.enable_vertex_attrib_array(position_attribute_location as u32);

but failed to also update

context.vertex_attrib_pointer_with_i32(0, 3, WebGl2RenderingContext::FLOAT, false, 0, 0);

to be

context.vertex_attrib_pointer_with_i32(position_attribute_location as u32, 3, WebGl2RenderingContext::FLOAT, false, 0, 0);

[Liamolucko]

Thanks!