[Core] External table documentation has a mistake

### Link to the page(s) on docs.getdbt.com requiring updates

https://docs.getdbt.com/reference/resource-properties/external?version=1.11

### Tell us more about this update

In the documentation it says the `expression` should be nested under `partitions.config.meta`.

However, it should be either nested under `partitions` or `partitions.config`. So a file should look like...

```
sources:
- name:
tables:
- name:
external:
location:
file_format:
row_format:
tbl_properties:
partitions:
- name:
data_type:
description:
meta: {dictionary}
expression:
- ...
:
```

https://github.com/dbt-labs/dbt-external-tables/blob/72df97ed2019c73d7d29b306c765ad78e3d81127/macros/plugins/snowflake/create_external_table.sql#L31

### Reviewers/Stakeholders/SMEs

.

### Related GitHub issues

_No response_

### Additional information

_No response_

Open the external tables page at the linked docs URL and compare its `expression` nesting with the referenced `create_external_table.sql` macro. Update the documentation example and wording to show the supported nesting, then verify the rendered page matches the documented structure.

Written by the indexing model from the issue text.