docs: Fix formatting consistency of CHANGELOG (#745)

### Changes

Minor updates to CHANGELOG.

### References

<!--
  Link to any associated issues, pull requests, or other resources.
-->

### Testing

<!--
  Tests must be added for new functionality.
  Existing tests should complete without errors.
  100% test coverage is required.
-->

### Contributor Checklist

- [x] I agree to adhere to the [Auth0 General Contribution
Guidelines](https://github.com/auth0/open-source-template/blob/master/GENERAL-CONTRIBUTING.md).
- [x] I agree to uphold the [Auth0 Code of
Conduct](https://github.com/auth0/open-source-template/blob/master/CODE-OF-CONDUCT.md).

Author: evansims

CHANGELOG.md

@@ -20,15 +20,15 @@
 
 ## [8.8.0](https://github.com/auth0/auth0-PHP/tree/8.8.0) - 2023-10-18
 
-### Added
+**Added**
 
 - Support `initiate_login_uri` property for `PATCH` requests to the `/api/v2/clients/:id` Management API endpoint. [#732](https://github.com/auth0/auth0-PHP/pull/732)
 
-### Fixed
+**Fixed**
 
 - Token generator typo in claim header. [#729](https://github.com/auth0/auth0-PHP/pull/729)
 
-### Changed
+**Changed**
 
 - `composer.json` now identifies `mbstring` as a required extension. [#730](https://github.com/auth0/auth0-PHP/pull/730)
 - PHP 8.3 nightlies have been added to the continuous integration testing matrix. [#733](https://github.com/auth0/auth0-PHP/pull/733)