If Tim Kring is ready to make some changes to the format of “Heroes” to bring viewers back, he better hope there’s a show to do it with. The NBC series earned a 4.6 rating/7 share, according to Fast National ratings from Nielsen Media Research, another record in “Heroes’” smallest audience book. With that rating, “Heroes” beat only “Prison Break” on Fox and “One Tree Hill” on The CW, and was murdered by “Dancing With the Stars” and “Samantha Who” on ABC, and CBS’ comedy block. That audience is 2 percent off the previous series low “Heroes” earned last week, and is nearly 25 percent off its season premiere audience, which already was considered low compared to its second season. The only highlight for “Heroes” is that it’s still NBC’s most-watched program on Mondays. Its …