I'm guessing this is a temporary issue (but an annoying one nonetheless and one that I dealt with this morning). That repository is most likely an additional one set alongside the "CRAN" repository as "CRANextra". The solution is to redefine the repos option to not include CRANextra. You can use the setRepositories() command and unselect "CRANextra". Alternatively you could just run the following code which will remove all repositories except the "CRAN" selection.
options(repos = getOption("repos")["CRAN"])
