Fix materialized view label (#6370)

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

The materialized view materialization is named `materialized_view` in
snake case, not `materialized view`, at least for the Postgres adapter.
The docs didn't reflect this.


## Checklist
- [x] I have reviewed 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.
- [x] The topic I'm writing about is for specific dbt version(s) and I
have versioned it according to the [version a whole
page](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
and/or [version a block of
content](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#versioning-blocks-of-content)
guidelines.
- [x] I have added checklist item(s) to this list for anything anything
that needs to happen before this PR is merged, such as "needs technical
review" or "change base branch."
- [ ] The content in this PR requires a dbt release note, so I added one
to the [release notes
page](https://docs.getdbt.com/docs/dbt-versions/dbt-cloud-release-notes).
- [ ] Check whether this is also true for adapters other than Postgres.

Co-authored-by: Leona B. Campbell <[email protected]>

website/docs/docs/build/materializations.md

@@ -111,7 +111,7 @@ When using the `table` materialization, your model is rebuilt as a <Term id="tab
 
 ### Materialized View
 
-The `materialized view` materialization allows the creation and maintenance of materialized views in the target database.
+The `materialized_view` materialization allows the creation and maintenance of materialized views in the target database.
 Materialized views are a combination of a view and a table, and serve use cases similar to incremental models.
 
 * **Pros:**