[DOCS] Fix incorrect use of "updateable" flag in synonyms documentation

[ganiamine]

Fixes #120865

This PR fixes the incorrect use of the updateable flag in the Apply synonyms at index or search time documentation.

The previous example incorrectly added the updateable flag to the field mapping, which results in a mapper_parsing_exception. The corrected example applies the updateable flag to the token filter configuration instead.

Changes:
Updated the example code snippet as follows:

Before:

  "mappings": {
    "properties": {
      "title": {
        "type": "text",
        "search_analyzer": "my_analyzer",
        "updateable": true
      }
    }
  }

After:

{
  "mappings": {
    "properties": {
      "title": {
        "type": "text",
        "search_analyzer": "my_analyzer"
      }
    }
  },
  "settings": {
    "analysis": {
      "analyzer": {
        "my_analyzer": {
          "tokenizer": "whitespace",
          "filter": [
            "synonyms_filter"
          ]
        }
      },
      "filter": {
        "synonyms_filter": {
          "type": "synonym",
          "synonyms_path": "analysis/synonym-set.txt",
          "updateable": true
        }
      }
    }
  }
}

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[carlosdelest]

Hey @ganiamine, thanks for opening this PR!

This change LGTM, thanks for the catch 👍 . You will need to sign the CLA for your PR to be accepted, once you do that I'll make sure this is merged.

[ganiamine]

Hello @carlosdelest,
I have already signed the CLA, but it seems the bot hasn't detected it yet.
Could someone assist me with this?

[leemthompo]

run docs-build

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@elasticmachine test this

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💚 Backport successful

Status	Branch	Result
✅	8.16
✅	8.17
✅	8.x