docs: Fix formatting consistency of CHANGELOG (#745)

### Changes

Minor updates to CHANGELOG.

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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)