update check cols all syntax to be correct (#6731)

## What are you changing in this pull request and why?

This is a really small change to setting your `check_cols` config in the
new yaml snapshot definitions correctly. The old version was
syntactically incorrect as putting `all` in an iterable like

```
check_cols:
  - all
```
caused dbt to end up executing the [elif
block](https://github.com/dbt-labs/dbt-adapters/blob/47b19b034984e144b174b268996c20f434f7139e/dbt/include/global_project/macros/materializations/snapshots/strategies.sql#L111)
when trying to ascertain the schema shape.

The correct syntax introduced here has dbt execute the [correct
line](https://github.com/dbt-labs/dbt-adapters/blob/47b19b034984e144b174b268996c20f434f7139e/dbt/include/global_project/macros/materializations/snapshots/strategies.sql#L108)
instead.

The current syntax leads a user to get the following error as dbt tries
to execute this introspective query as it treats `all` like it's a
column name:
```
select * from (
        select all from (
                select * from database.schema.snapshot
            ) subq
    ) as __dbt_sbq
    where false
    limit 0
20:13:50 Snowflake adapter: Snowflake query id: xxx
20:13:50 Snowflake adapter: Snowflake error: xxx (xxx): xxx: SQL compilation error:
syntax error line 2 at position 19 unexpected 'from'.
20:13:50 Database Error in snapshot snapshot_name (snapshots/snapshot.yml)
  xxx (xxx): xxx: SQL compilation error:
  syntax error line 2 at position 19 unexpected 'from'.
```

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<!-- vercel-deployment-preview -->
---
🚀 Deployment available! Here are the direct links to the updated files:


-
https://docs-getdbt-com-git-trouze-check-cols-all-dbt-labs.vercel.app/reference/resource-configs/check_cols

<!-- end-vercel-deployment-preview -->

website/docs/reference/resource-configs/check_cols.md

@@ -122,8 +122,7 @@ snapshots:
       schema: snapshots
       unique_key: id
       strategy: check
-      check_cols:
-        - all
+      check_cols: all
   ```
 </File>