Fix the gnt-instance section in the htools manpage

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>

man/htools.rst

@@ -181,7 +181,7 @@ support all options. Some common options are:
   - instance disk size
   - instance vcpus
   - instance status (in Ganeti's format, e.g. ``running`` or ``ERROR_down``)
-  - instance ``auto_balance`` flag (see man page **gnt-instance** (7))
+  - instance ``auto_balance`` flag (see man page **gnt-instance**\(8))
   - instance primary node
   - instance secondary node(s), if any
   - instance disk type (e.g. ``plain`` or ``drbd``)