Fix incorrect object:_field() usage docs

[vallode]

Looks like ref.field(name) was not documented properly.

static int meta_struct_field_reference(lua_State *state)
{
    if (lua_gettop(state) != 2)
        luaL_error(state, "Usage: object._field(name)");

Both the struct and container versions require self (in Lua terms). Running either with the provided syntax results in an error prompting Usage: object._field(index) which is also incorrect.