Typo

Testing.Rmd

@@ -1363,7 +1363,7 @@ test_that("foofy() does this", {
 If it's easy to avoid writing files from you tests, that is definitely the best plan.
 But there are many times when you really must write files.
 
-**You should only write file inside the session temp directory.**
+**You should only write files inside the session temp directory.**
 Do not write into your package's `tests/` directory.
 Do not write into the current working directory.
 Do not write into the user's home directory.