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/nfs/nfs-rdma.txt b/Documentation/filesystems/nfs/nfs-rdma.txt
index 1e65645..22dc0dd 100644
--- a/Documentation/filesystems/nfs/nfs-rdma.txt
+++ b/Documentation/filesystems/nfs/nfs-rdma.txt
@@ -110,10 +110,10 @@
   - Install a Linux kernel with NFS/RDMA
 
     The NFS/RDMA client and server are both included in the mainline Linux
-    kernel version 2.6.25 and later. This and other versions of the 2.6 Linux
+    kernel version 2.6.25 and later. This and other versions of the Linux
     kernel can be found at:
 
-    ftp://ftp.kernel.org/pub/linux/kernel/v2.6/
+    https://www.kernel.org/pub/linux/kernel/
 
     Download the sources and place them in an appropriate location.