Update README.md (#98)

Author: jasonfb

README.md

@@ -42,7 +42,7 @@ to 1 of the 3 viable ways of loading ES Module javascript packages.
 For example:
 
 ```rb
-# config/importmaps.rb
+# config/importmap.rb
 pin "react", to: "https://ga.jspm.io/npm:[email protected]/index.js"
 ```