Update vectors.Rmd (#929)

Section 20.4.5 Subsetting
A small grammatical issue on line 329
(insert the letter 'a')

vectors.Rmd

@@ -326,7 +326,7 @@ Named vectors are most useful for subsetting, described next.
 ### Subsetting {#vector-subsetting}
 
 So far we've used `dplyr::filter()` to filter the rows in a tibble.
-`filter()` only works with tibble, so we'll need new tool for vectors: `[`.
+`filter()` only works with tibble, so we'll need a new tool for vectors: `[`.
 `[` is the subsetting function, and is called like `x[a]`.
 There are four types of things that you can subset a vector with: