Add a clear example of the REST api call to update a role [DOC]

[AntonEliatra]

Current documentation is incorrect on rest API calls to patch roles.
current docs here https://opensearch.org/docs/latest/security/access-control/api/#patch-role

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Current PATCH method specified below is incorrect and returns 500 error, with details below:
java.lang.UnsupportedOperationException: JsonNode not of type ObjectNode (but com.fasterxml.jackson.databind.node.TextNode), cannot call withArray() on it

To test, try to run the example in the docs:
PATCH _plugins/_security/api/roles [ { "op": "replace", "path": "/role1/index_permissions/0/fls", "value": ["test1", "test2"] }, { "op": "remove", "path": "/role1/index_permissions/0/dls" }, { "op": "add", "path": "/role2/cluster_permissions", "value": ["manage_snapshots"] } ]