[DOCS] Update elasticsearch-certutil example (#61110)

docs/reference/commands/certutil.asciidoc

@@ -290,7 +290,7 @@ parameter to specify the location of the file. For example:
 
 [source, sh]
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-bin/elasticsearch-certutil cert --silent --in instances.yml --out test1.zip --pass testpassword
+bin/elasticsearch-certutil cert --silent --in instances.yml --out test1.zip --pass testpassword --keep-ca-key
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 This command generates a compressed `test1.zip` file. After you decompress the