mkswap: use dd(1) in example rather than fallocate(1)

The man page warns against fallocate on some filesystems and recommends
dd(1) as the most portable solution. So, let's use dd(1) also in the
example used in the same man page.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1203378
Signed-off-by: Karel Zak <kzak@redhat.com>

disk-utils/mkswap.8

@@ -124,10 +124,12 @@ e.g.\& using a command like
 
 .nf
 .RS
-# fallocate \-\-length 8GiB swapfile
+# dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024))
 .RE
 .fi
 
+to create 8GiB swapfile.
+
 Note that a swap file must not contain any holes.  Using
 .BR cp (1)
 to create the file is not acceptable.  Neither is use of