[plt-scheme] find-executable-path docs typo?

Hi,

In find-executable-path documentation:
(find-executable-path	 	program-sub	 	 	 	
 	 	related-sub	 	 	 	
 	 [	deepest?])	 	→	 	(or/c path? #f)
  program-sub : path-string?
  related-sub : path-string?
  deepest? : any/c = #f

However, I can:
(find-executable-path "which")

which means that the second argument is also optional but that doesn't
seem to be conveyed by the documentation.

Cheers,

-- 
Paulo Jorge Matos - pocmatos at gmail.com
Webpage: http://www.personal.soton.ac.uk/pocm