qapi/yank: Tweak @yank's error description for consistency

Phrase it like "If <condition>, <error>", like we do elsewhere.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240227113921.236097-8-armbru@redhat.com>
diff --git a/qapi/yank.json b/qapi/yank.json
index fffb39a..89f2f4d 100644
--- a/qapi/yank.json
+++ b/qapi/yank.json
@@ -79,7 +79,7 @@
 # @instances: the instances to be yanked
 #
 # Errors:
-#     - @DeviceNotFound error, if any of the YankInstances doesn't exist
+#     - If any of the YankInstances doesn't exist, DeviceNotFound
 #
 # Example:
 #