Fix typo

Doc/lib/libpkgutil.tex

@@ -30,7 +30,7 @@ __path__ = extend_path(__path__, __name__)
   with \code{import}.  A \file{*.pkg} file is trusted at face value:
   apart from checking for duplicates, all entries found in a
   \file{*.pkg} file are added to the path, regardless of whether they
-  exist the filesystem.  (This is a feature.)
+  exist on the filesystem.  (This is a feature.)
 
   If the input path is not a list (as is the case for frozen
   packages) it is returned unchanged.  The input path is not