man: Fix description of getopts when last argument reached

The description of getops in the manual incorrectly states that
var will be set to "--" when no arguments remain.  In fact it
will be set to "?".

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

src/dash.1

@@ -1402,14 +1402,9 @@ By specifying a colon as the first character of
 .Va optstring
 all errors will be ignored.
 .Pp
-A nonzero value is returned when the last option is reached.
-If there are no remaining arguments,
+After the last option
 .Ic getopts
-will set
-.Va var
-to the special option,
-.Dq -- ,
-otherwise, it will set
+will return a non-zero value and set
 .Va var
 to
 .Dq \&? .