Correct small typo in intro.Rmd (hadley#492)

Last sentence in section 1.4.1: R should be changed to "putting it off only makes it worse" or "putting it off will only make it worse".

intro.Rmd

@@ -103,7 +103,7 @@ There are four things you need to run the code in this book: R, RStudio, a colle
 
 To download R, go to CRAN, the **c**omprehensive **R** **a**rchive **n**etwork. CRAN is composed of a set of mirror servers distributed around the world and is used to distribute R and R packages. Don't try and pick a mirror that's close to you: instead use the cloud mirror, <https://cloud.r-project.org>, which automatically figures it out for you.
 
-A new major version of R comes out once a year, and there are 2-3 minor releases each year. It's a good idea to update regularly. Upgrading can be a bit of a hassle, especially for major versions, which require you to reinstall all your packages, but putting it off only make it worse.
+A new major version of R comes out once a year, and there are 2-3 minor releases each year. It's a good idea to update regularly. Upgrading can be a bit of a hassle, especially for major versions, which require you to reinstall all your packages, but putting it off only makes it worse.
 
 ### RStudio