Correct typo in "Reproducing the book" section

Found a typo; suggesting a fix! ;)

README.Rmd

@@ -65,7 +65,7 @@ This impact is recorded in [`our-impact.csv`](https://github.com/Robinlovelace/g
 ## Reproducing the book
 
 To ease reproducibility, we created the `geocompkg` package.
-Installing it from GitHub will install all the R packages needed build the book (you will a computer with necessary [system dependencies](https://github.com/r-spatial/sf#installing) and the [**remotes**](https://github.com/r-lib/remotes/) package installed):
+Installing it from GitHub will install all the R packages needed build the book (you will need a computer with necessary [system dependencies](https://github.com/r-spatial/sf#installing) and the [**remotes**](https://github.com/r-lib/remotes/) package installed):
 
 
 ```{r, eval=FALSE, message=FALSE}