Documentation: Fix dead URLs to ftp.kernel.org

As ftp.kernel.org is closed [0], this commit fixes dead URLs in
documents to use www.kernel.org instead.

[0] https://www.kernel.org/shutting-down-ftp-services.html

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Acked-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 3698ed3..5a8f7f4 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -25,7 +25,7 @@
 	
 	or
 
-    ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
+    https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
 
 	or grab the latest git repository from: