Fix typo in readme

Author: JoviDeCroock

README.md

@@ -103,7 +103,7 @@ graphql({ schema, source }).then((result) => {
 });
 ```
 
-**Note**: Please don't forget to set `NODE_ENV=production` if you are running a production server. It will disable some checks that can be useful during development but will significantly improve performance.
+**Note**: Please don't forget to set `NODE_ENV=production` if you are running a production server. It will disable some checks that can be useful during development but will significantly impact performance.
 
 ## Want to ride the bleeding edge?