[boost] [program_options] typo in doc

http://www.boost.org/doc/html/program_options/overview.html#id1247613 has the line pd.add("output-file", 2).add_optional("input-file", -1); which is not supported. Only void add(...) exists. pd.add("output-file", 2) pd.add("input-file", -1); would be correct I think. matt.