[DOCS] Change index name in rollover ILM example (#47492)

The warning section above the example tells that index name has to end with the digits but the example itself uses index name without digits which is confusing.

Author: immon

docs/reference/ilm/policy-definitions.asciidoc

@@ -368,12 +368,12 @@ follower index into a regular index as if <<ilm-unfollow-action,the Unfollow
 action>> had been used instead of rolling over.
 
 For example, if an index to be managed has an alias `my_data`. The managed
-index "my_index" must be the write index for the alias. For more information, read
+index "my_index-000001" must be the write index for the alias. For more information, read
 <<indices-rollover-is-write-index,Write Index Alias Behavior>>.
 
 [source,js]
 --------------------------------------------------
-PUT my_index
+PUT my_index-000001
 {
   "settings": {
     "index.lifecycle.name": "my_policy",