Fix materialized view label

[skion]

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

• I have reviewed the Content style guide so my content adheres to these guidelines.
• The topic I'm writing about is for specific dbt version(s) and I have versioned it according to the version a whole page and/or version a block of content guidelines.
• 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.
• Check whether this is also true for adapters other than Postgres.

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Name	Status	Preview	Updated (UTC)
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[runleonarun]

Great catch @skion! And thank you for creating a PR to fix it. I will get this merged!

[runleonarun]

LGTM!