Fix typo

Closes GH-11.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

readme.md

@@ -62,7 +62,7 @@ Alternatively, you can also use the underlying features separately:
 When you need a syntax tree, combine this package with
 [`mdast-util-gfm`][mdast-util-gfm].
 
-All these packages are used in [`remark-gfm`][remark-gfm], which focusses on
+All these packages are used in [`remark-gfm`][remark-gfm], which focuses on
 making it easier to transform content by abstracting these internals away.
 
 ## Install