Brian Williams marked 20 years at NBC by talking to Jonathan Tisch in front of a crowd at the 92nd Street Y — but admitted he won’t give his daughter Allison Williams, a star of HBO’s “Girls,” advice on managing fame. He said, “She got through Yale. I only have 18 [college] credits. I think I could actually do harm to a perfectly promising career if I try to give her advice.” But he admitted he was proud, saying, “We’re Ma and Pa Kettle sitting at home watching TV, and there is Allison sitting in the front row at the Grammys.” Of his start in news, Williams joked, “My mother told me that I could do a better job than the reporters on TV. Not sure how she came to that conclusion, since I was working at a pancake house at the time.”