Amend notebook.source actions on save recommendation (#380)

Looks like this line got prefixed with `notebook` despite the setting
targeting non-notebook files. Just a one-line change, so I hope it's
alright I didn't raise an issue on it. Fixes what looks like a one-line
error introduced in #379?

README.md

@@ -135,7 +135,7 @@ Similarly, you can configure Ruff to organize imports on-save by enabling the
 {
   "[python]": {
     "editor.codeActionsOnSave": {
-      "notebook.source.organizeImports": true
+      "source.organizeImports": true
     }
   }
 }