corrected argument to `install_github`

The correct argument to `install_github` here is actually `reshape` rather than `reshape2` (this is a little confusing!):

    devtools::install_github("hadley/reshape2")
    ## Installing github repo reshape2/master from hadley
    ## Downloading master.zip from https://github.com/hadley/reshape2/archive/master.zip
    ## Error in (function (url, name = NULL, subdir = NULL, config = list(),  : 
    ## client error: (404) Not Found