block: Correct documentation for BLOCK_WRITE_THRESHOLD

Use the correct command name.
Signed-off-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qapi/block-core.json

@@ -3624,7 +3624,7 @@
 # means the device should be extended to avoid pausing for
 # disk exhaustion.
 # The event is one shot. Once triggered, it needs to be
-# re-registered with another block-set-threshold command.
+# re-registered with another block-set-write-threshold command.
 #
 # @node-name: graph node name on which the threshold was exceeded.
 #