Apply suggestions from code review

Co-authored-by: Sarah Chavis <62406755+schavis@users.noreply.github.com>

Author: rculpepper

content/vault/v1.21.x (rc)/content/api-docs/system/billing-pki-certificate-count.mdx

@@ -2,32 +2,31 @@
 layout: api
 page_title: /sys/billing/certificates - HTTP API
 description: >-
-  The `/sys/billing/certificates` endpoint is used to return data about the number of PKI certificates issued by Vault.
+  Use the `/sys/billing/certificates` endpoint to fetch data about the number of PKI certificates issued by Vault.
 ---
 
 `# /sys/billing/certificates`
 
-The `/sys/billing/certificates` endpoint is used to return data about the number of PKI certificates issued by Vault.
-It returns information for the entire cluster.
+Use the `/sys/billing/certificates` endpoint to fetch data about the
+number of PKI certificates issued by Vault. for a cluster.
 
 ## PKI Certificate Counts
 
-This endpoint returns the number of PKI certificates issued by Vault in each month of the given time range.
-If there are no counts for a given month, an error will be returned for that month.
+The Certificate Counts endpoint returns the number of PKI certificates issued by Vault, or an error, for each month of the given time range.
 
 | Method | Path                        |
 | :----- | :-------------------------- |
 | `GET`  | `/sys/billing/certificates` |
 
-### Request Parameters
+### Request parameters
 
 - `start_time` `(string: <required>)` - The first month for which to return certificate counts.
 The time must be a timestamp in RFC 3339 format (e.g., `2025-01-01T00:00:00Z` for January 2025).
 
 - `end_time` `(string: <required>)` - The last month for which to return certificate counts. The `end_time` must not be before the `start_time`.
 The time must be a timestamp in RFC 3339 format (e.g., `2025-01-01T00:00:00Z` for January 2025).
 
-### Sample Payload
+### Sample payload
 
 ```json
 {