Switch SCD example to entity type natural (#4629)

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

This changes the entity type for the SCD example to `natural`.

We recommend using the `natural` type for what is normally the `primary`
entity in an SCD semantic model layout. The distinction is that
`primary` indicates one row per entity value, while `natural` does not,
and in an SCD context there is one row per entity value/time window set,
rather than one row per entity value as in standard conformed dimension
tables.

## 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.
- [x] Add a checklist item for anything that needs to happen before this
PR is merged, such as "needs technical review" or "change base branch."

website/docs/docs/build/dimensions.md

@@ -252,7 +252,8 @@ This example shows how to create slowly changing dimensions (SCD) using a semant
 | 333             | 2    | 2020-08-19 | 2021-10-22| 
 | 333             | 3    | 2021-10-22 | 2048-01-01|  
 
-Take note of the extra arguments under `validity_params`: `is_start` and `is_end`. These arguments indicate the columns in the SCD table that contain the start and end dates for each tier (or beginning or ending timestamp column for a dimensional value).
+
+The `validity_params` include two important arguments — `is_start` and `is_end`. These specify the columns in the SCD table that mark the start and end dates (or timestamps) for each tier or dimension. Additionally, the entity is tagged as `natural` to differentiate it from a `primary` entity. In a `primary` entity, each entity value has one row. In contrast, a `natural` entity has one row for each combination of entity value and its validity period.
 
 ```yaml 
 semantic_models:
@@ -280,9 +281,11 @@ semantic_models:
       - name: tier
         type: categorical
 
+    primary_entity: sales_person
+
     entities:
       - name: sales_person
-        type: primary 
+        type: natural 
         expr: sales_person_id
 ```