QMP: Fix blockdev-snapshot-sync doc example

Fix wrong command name.
Reported-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

qmp-commands.hx

@@ -710,10 +710,10 @@ Arguments:
 
 Example:
 
--> { "execute": "blockdev-snapshot", "arguments": { "device": "ide-hd0",
-                                                    "snapshot-file":
-                                                    "/some/place/my-image",
-                                                    "format": "qcow2" } }
+-> { "execute": "blockdev-snapshot-sync", "arguments": { "device": "ide-hd0",
+                                                         "snapshot-file":
+                                                        "/some/place/my-image",
+                                                        "format": "qcow2" } }
 <- { "return": {} }
 
 EQMP