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
Merge pull request #53 from benmarwick/patch-1

corrected argument to `install_github`