remove mention of arbitrary script from teh readme

Author: DannyBen

README.md

@@ -217,8 +217,7 @@ these (whichever exists):
 
 You can use the built in completions script tester by running `completely test`.
 
-This command lets you test any completions script, whether it was generated by
-completely or not.
+This command lets you test completions for your completions script.
 
 In addition, you can set the `COMPLETELY_DEBUG` environment variable to any value
 in order to generate scripts with some additional debugging functionality. Run