Fix the example for api.Relation.create() (#5249)

[Preview](https://docs-getdbt-com-git-dbeatty10-patch-3-dbt-labs.vercel.app/guides/create-new-materializations?step=2#update-the-relation-cache)

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

### Problem

The code example within the guide for [custom
materializations](https://docs.getdbt.com/guides/create-new-materializations?step=2#update-the-relation-cache)
related to
[`api.Relation.create`](https://docs.getdbt.com/reference/dbt-classes#using-relations)
doesn't work as-is and yields a confusing error message:

```
23:15:15    Field "path" of type Path in PostgresRelation has invalid value {'database': 'my_database', 'schema': 'my_schema', 'identifier': Undefined}
```

### Solution

For some reason, `identifier` needs to be prefixed with `this.` whereas
`database` and `schema` don't 🤷

To make the code example most readable and least confusing, I'm opting
to add this prefix to all three.

### Alternative solutions

#### Alternative 1
Alternatively, we could have switched to using the approach in the
current
[table](https://github.com/dbt-labs/dbt-adapters/blob/7392debf7ff1aa5d58c0fe26eb76ac8ade4fe15b/dbt/include/global_project/macros/materializations/models/table.sql#L4)
and
[view](https://github.com/dbt-labs/dbt-adapters/blob/7392debf7ff1aa5d58c0fe26eb76ac8ade4fe15b/dbt/include/global_project/macros/materializations/models/view.sql#L4)
that uses `this.incorporate` instead:

```sql
target_relation = this.incorporate(type='...')
```

#### Alternative 2

Only replace `identifier=identifier` with `identifier=this.identifier`

#### Alternative 3

Only replace `identifier=identifier` with `identifier=model['alias']`

#### Alternative 4

Only add `{%- set identifier = model['alias'] -%}` earlier in the macro.

I opted against all the alternatives in order to use a consistent
pattern across `database`, `schema`, and `identifier` and also to make
the fewest changes possible.

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.

Co-authored-by: Mirna Wong <[email protected]>

website/docs/guides/create-new-materializations.md

@@ -125,7 +125,7 @@ Materializations should [return](/reference/dbt-jinja-functions/return) the list
 {%- materialization my_view, default -%}
 
   {%- set target_relation = api.Relation.create(
-        identifier=identifier, schema=schema, database=database,
+        identifier=this.identifier, schema=this.schema, database=this.database,
         type='view') -%}
 
   -- ... setup database ...
@@ -187,4 +187,4 @@ Specific materializations can be selected by using the dot-notation when selecti
 
 We recommend _not_ overriding materialization names directly, and instead using a prefix or suffix to denote that the materialization changes the behavior of the default implementation (eg. my_project_incremental).
 
-</div>
+</div>