[6.8] [DOCS] Fixes ML get scheduled events API (#78809)

[lcawl]

Backports the following commits to 6.8:

• [DOCS] Fixes ML get scheduled events API ([DOCS] Fixes ML get scheduled events API #78809)

Also removes request body, since in tests on 6.8.19, any request objects such as:

GET _xpack/ml/calendars/planned-outages/events
{
"page":{
"from": 0,
"size": 5
},
"start": "1640390300000",
"end": "1640476900000"
}

Returns errors:

{
"error": {
"root_cause": [
{
"type": "x_content_parse_exception",
"reason": "[2:3] [cluster:admin/xpack/ml/calendars/events/post] unknown field [page], parser not found"
}
],
"type": "x_content_parse_exception",
"reason": "[2:3] [cluster:admin/xpack/ml/calendars/events/post] unknown field [page], parser not found"
},
"status": 400
}