it is 'data frame'

git-svn-id: https://svn.r-project.org/R/trunk@63966 00db46b3-68df-0310-9c12-caf00c1e9a41
clarify, spell data frame not dataframe

git-svn-id: https://svn.r-project.org/R/trunk@63967 00db46b3-68df-0310-9c12-caf00c1e9a41