Fix typo

Closes GH-73.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

readme.md

@@ -398,7 +398,7 @@ this functionality, which defaulted to true.
 
 ## Security
 
-Use of `remark-frontmatter` does not involve **[rehype][]** ([hast][]) or user
+Use of `remark-gfm` does not involve **[rehype][]** ([hast][]) or user
 content so there are no openings for [cross-site scripting (XSS)][wiki-xss]
 attacks.