typo, and correct arg to read.csv is na.strings

one of two consecutive "using"s removed

From the help for read.csv:
na.strings
a character vector of strings which are to be interpreted as NA values. Blank fields are also considered to be missing values in logical, integer, numeric and complex fields.
Merge pull request #239 from ajschumacher/patch-8

typo, and correct arg to read.csv is na.strings