[Rd] minor typo in by.Rd (PR#7211)

[deepayan at stat.wisc.edu]

src/library/base/man/by.Rd has 


\item{INDICES}{a factor or a list of factors, each of 
   length \code{nrow(x)}.}
\item{FUN}{a function to be applied to data frame 
   subsets of \code{x}.}


'x' should be 'data' in both.


Also, in 


\details{
  A data frame is split by row into data frames
  subsetted by the values of one or more factors, and function
  \code{FUN} is applied to each subset in term.


the last word was probably meant to be 'turn'.


-Deepayan