network: fix ambiguous "without mode" wording in docs (#42428)

- In `systemd.network.xml`, replaced `"without mode"` with `"without static"`
to clarify that if an IPv6 address is specified without the explicit keyword
`static`, then `static` mode is assumed.
- The original wording was ambiguous because "mode" appears multiple
times in the surrounding context (referring to IPv6 link-local address
modes like `eui64`, `static`, etc.).

Fixes: #39754

man/systemd.network.xml

@@ -3517,7 +3517,7 @@ MultiPathRoute=2001:db8::1@eth0</programlisting>
                   generated by using the EUI-64 algorithm. Because the interface identifier is static,
                   if Duplicate Address Detection detects that the computed address is a duplicate
                   (in use by another node on the link), then this mode will fail to provide an address
-                  for that prefix. If an IPv6 address without mode is specified, then
+                  for that prefix. If an IPv6 address without <literal>static</literal> is specified, then
                   <literal>static</literal> mode is assumed.
                 </para>