doc: fix qmp event type

Event name for hot unplug errors was wrong.
Make doc match code.

Cc: Zhu Guihua <zhugh.fnst@cn.fujitsu.com>
Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/qmp/qmp-events.txt

diff --git a/docs/qmp/qmp-events.txt b/docs/qmp/qmp-events.txt
index 6dc2cca..4c13d48 100644
--- a/docs/qmp/qmp-events.txt
+++ b/docs/qmp/qmp-events.txt
@@ -232,7 +232,7 @@
 { "event": "GUEST_PANICKED",
      "data": { "action": "pause" } }
 
-MEM_HOT_UNPLUG_ERROR
+MEM_UNPLUG_ERROR
 --------------------
 Emitted when memory hot unplug error occurs.
 
@@ -243,7 +243,7 @@
 
 Example:
 
-{ "event": "MEM_HOT_UNPLUG_ERROR"
+{ "event": "MEM_UNPLUG_ERROR"
   "data": { "device": "dimm1",
             "msg": "acpi: device unplug for unsupported device"
   },