Fix disk allocator docs
This commit fixes the disk allocator docs which were broken due to the
inadvertent removal of some docs snippet markup.
@@ -29,7 +29,8 @@ file or updated dynamically on a live cluster with the
the node.
`cluster.routing.allocation.disk.watermark.floodstage`::
-
++
+--
Controls the flood stage watermark. It defaults to 95%, meaning ES enforces
a read-only index block (`index.blocks.read_only_allow_delete`) on every
index that has one or more shards allocated on the node that has at least
