docs: fix recommended command line to generate site seed

Without -v, hexdump would sometimes replace duplicate bytes with "*" and
a newline.

docs/site-example/site.conf

@@ -17,7 +17,7 @@
 
   -- 32 bytes of random data, encoded in hexacimal
   -- Must be the same of all nodes in one mesh domain
-  -- Can be generated using: echo $(hexdump -n 32 -e '1/1 "%02x"' </dev/urandom)
+  -- Can be generated using: echo $(hexdump -v -n 32 -e '1/1 "%02x"' </dev/urandom)
   site_seed = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx',
 
   -- Prefixes used within the mesh.

docs/user/site.rst

@@ -30,7 +30,7 @@ site_seed
     The recommended way to generate a value for a new site is:
     ::
 
-        echo $(hexdump -n 32 -e '1/1 "%02x"' </dev/urandom)
+        echo $(hexdump -v -n 32 -e '1/1 "%02x"' </dev/urandom)
 
 prefix4 \: optional
     The IPv4 Subnet of your community mesh network in CIDR notation, e.g.