Fix typo in gatsby-remark-katex README (#3342)

Author: dominicchapman

packages/gatsby-remark-katex/README.md

@@ -5,7 +5,7 @@
 
 ## Install
 
-`npm install --save gatsby-transformer-remark atsby-remark-katex`
+`npm install --save gatsby-transformer-remark gatsby-remark-katex`
 
 ## How to use