fb_ethtool: fix README to reference correct node attribute

Summary:
Fix README to reference correct node attribute

Reviewed By: epilatow

Differential Revision: D20773603

fbshipit-source-id: fd8c67fdcf8b27af386e7999f3b57111dd7c9b89

cookbooks/fb_ethtool/README.md

@@ -12,4 +12,4 @@ Attributes
 Usage
 -----
 Include `fb_ethtool` in your runlist to use it. You can opt out of package
-management by settings `node['fb_screen']['manage_packages']` to `false`.
+management by setting `node['fb_ethtool']['manage_packages']` to `false`.