[DOCS] Replace nested open block for Asciidoctor migration (elastic#4…

…1168)

* [DOCS] Fix nested open blocks for Asciidoctor migration

* [DOCS] Reformat table to definitions

docs/reference/index-modules.asciidoc

@@ -45,26 +45,17 @@ specific index module:
     part of the cluster.
 
 `index.shard.check_on_startup`::
-+
---
-Whether or not shards should be checked for corruption before opening. When
-corruption is detected, it will prevent the shard from being opened. Accepts:
-
-`false`::
-
-    (default) Don't check for corruption when opening a shard.
-
-`checksum`::
 
-    Check for physical corruption.
-
-`true`::
-
-    Check for both physical and logical corruption. This is much more
-    expensive in terms of CPU and memory usage.
-
-WARNING: Expert only. Checking shards may take a lot of time on large indices.
---
+Whether or not shards should be checked for corruption before opening. When
+corruption is detected, it will prevent the shard from being opened.
+Accepts:
+`false`::: (default) Don't check for corruption when opening a shard.
+`checksum`::: Check for physical corruption.
+`true`::: Check for both physical and logical corruption. This is much more
+expensive in terms of CPU and memory usage.
++
+WARNING: Expert only. Checking shards may take a lot of time on large
+indices.
 
 [[index-codec]] `index.codec`::