docs: fixed Moving Sliding Average (per 10 days) query typo

[juunone]

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Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

• fixed Moving Sliding Average (per 10 days) query typo
  • AS keyword missed

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[vdimir]

Actually AS keyword is not required here, we do support such syntax.

You can define the alias for a table name or a column name in a SELECT clause without using the AS keyword.
https://clickhouse.com/docs/en/sql-reference/syntax/#expression-aliases

Anyway, thank you for your change, it's a bit easier to read, so let's merge it.