commands: correct documentation of hg serve’s --ipv6 option

When the --ipv6 option is given, the server doesn’t listen to a IPv4 socket.
This can be verified by running two servers, one with and one
without the option, which works fine.

I think that listening to both a IPv4 and a IPv6 socket would be better,
but given that the Python standard library class underlying the
HTTP server supports only one socket, this is not trivial.

mercurial/commands.py

@@ -6602,7 +6602,7 b' def root(ui, repo, **opts):'
         ),
         (b't', b'templates', b'', _(b'web templates to use'), _(b'TEMPLATE')),
         (b'', b'style', b'', _(b'template style to use'), _(b'STYLE')),
-        (b'6', b'ipv6', None, _(b'use IPv6 in addition to IPv4')),
+        (b'6', b'ipv6', None, _(b'use IPv6 instead of IPv4')),
         (b'', b'certificate', b'', _(b'SSL certificate file'), _(b'FILE')),
         (b'', b'print-url', None, _(b'start and print only the URL')),
     ]