Fix 2 typos in OO-tradeoffs.Rmd (hadley#871)

"is" to "if" and remove unnecessary "because". I assign the copyright in this edit to Hadley Wickham.

OO-tradeoffs.Rmd

@@ -40,7 +40,7 @@ knitr::include_graphics("diagrams/s4-matrix-dsparseMatrix.png", dpi = 300)
 
 This domain is a good fit for S4 because there are often computational shortcuts for specific types of sparse matrix. S4 makes it easy to provide a general method that works for all inputs, and then provide a more specialised methods where the pair of data structures allow for a more efficient implementation. This requires careful planning to avoid method dispatch ambiguity, but the planning pays off for complicated systems.
 
-The biggest challenge to using S4 is the combination of increased complexity and absence of a single source of documentation. S4 is a complex system and it can be challenging to use effectively in practice. This wouldn't be such a problem is S4 documentation wasn't scattered through R documentation, books, and websites. S4 needs a book length treatment, but that book does not (yet) exist. (The documentation for S3 is no better, but because the lack is less painful because S3 is much simpler.)
+The biggest challenge to using S4 is the combination of increased complexity and absence of a single source of documentation. S4 is a complex system and it can be challenging to use effectively in practice. This wouldn't be such a problem if S4 documentation wasn't scattered through R documentation, books, and websites. S4 needs a book length treatment, but that book does not (yet) exist. (The documentation for S3 is no better, but the lack is less painful because S3 is much simpler.)
 
 ## R6 vs S3  {#s3-r6}