docs: fix list of domain states in virsh manpage

The virsh manpage lists "shutdown" and "dying" as two of the possible
domain states that could be listed in the output of the "virsh list"
command. However, a domain that is being shutdown will be listed as
"in shutdown", and the "dying" state doesn't even exist (and never
has, as far as I can tell from looking through git history - it was
shown in the original import of the virsh.pod file in 2006; there was
no VIR_DOMAIN_DYING state then, there wasn't one when those lines of
virsh.pod were tweaked in 2008, and there still isn't one
today. Apparently it was just something that sounded like a good idea
to someone at some time, but was never implemented...)

Resolves: https://bugzilla.redhat.com/1408778

tools/virsh.pod

@@ -440,7 +440,7 @@ running B<virsh suspend>.  When in a paused state the domain will still
 consume allocated resources like memory, but will not be eligible for
 scheduling by the hypervisor.
 
-=item B<shutdown>
+=item B<in shutdown>
 
 The domain is in the process of shutting down, i.e. the guest operating system
 has been notified and should be in the process of stopping its operations
@@ -457,11 +457,6 @@ The domain has crashed, which is always a violent ending.  Usually
 this state can only occur if the domain has been configured not to
 restart on crash.
 
-=item B<dying>
-
-The domain is in process of dying, but hasn't completely shutdown or
-crashed.
-
 =item B<pmsuspended>
 
 The domain has been suspended by guest power management, e.g. entered