virsh: Modify description for LIBVIRT_DEBUG=4 in manpage

As LIBVIRT_DEBUG=4 logs only error messages and there
are no levels above it, adjusting the description in
the man page accordingly.
Signed-off-by: NNitesh Konkar <nitkon12@linux.vnet.ibm.com>

tools/virsh.pod

@@ -4564,7 +4564,7 @@ Messages at level WARNING or above
 
 =item * LIBVIRT_DEBUG=4
 
-Messages at level ERROR or above
+Messages at level ERROR
 
 =back