wcsdup.3: RETURN VALUE: fix error in return value description

Return value for failure was accidentally changed from NULL to
-1 in man-pages commit 572acb41c4.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/wcsdup.3

@@ -60,7 +60,7 @@ and should be freed with
 On success,
 .BR wcsdup ()
 returns a pointer to the new wide-character string.
-On error, it returns \-1, with
+On error, it returns NULL, with
 .I errno
 set to indicate the cause of the error.
 .SH ERRORS