README: Fix instructions for .gitignore for local preview

When using netlify dev to locally preview the application, the 
out_publish/ and out_functions/ folders as well as the 404.html file 
should be added to .gitignore.

Author: FinnWoelm

README.md

@@ -105,9 +105,9 @@ Lastly, add the following lines to your `.gitignore`:
 # .gitignore
 
 # Files generated by next-on-netlify command
-functions/nextRouter
-out/
-404.html
+/out_publish/
+/out_functions/
+/404.html
 ```
 
 Now you're all set.