qapi: Clean up "Returns" sections

Drop list markup, since there's just one item left.

Drop "on success" where it is redundant with "Returns:".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240227113921.236097-7-armbru@redhat.com>
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 94b01de..1874f88 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2457,7 +2457,7 @@
 # @unstable: This command is meant for debugging.
 #
 # Returns:
-#     - BlockDirtyBitmapSha256 on success
+#     BlockDirtyBitmapSha256
 #
 # Errors:
 #     - If @node is not a valid block device, DeviceNotFound
@@ -6084,7 +6084,7 @@
 # @name: optional the snapshot's name to be deleted
 #
 # Returns:
-#     - SnapshotInfo on success
+#     SnapshotInfo
 #
 # Errors:
 #     - If @device is not a valid block device, GenericError