docs: fix changelog header to consistent size (#1404)

Author: dandhlee

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Changelog
 
-### [1.13.1](https://github.com/googleapis/python-aiplatform/compare/v1.13.0...v1.13.1) (2022-05-26)
+## [1.13.1](https://github.com/googleapis/python-aiplatform/compare/v1.13.0...v1.13.1) (2022-05-26)
 
 
 ### Features
@@ -60,7 +60,7 @@
 
 * fix type in docstring for map fields ([847ad78](https://github.com/googleapis/python-aiplatform/commit/847ad789e09aec14238a7476a3fa88729ce24d6f))
 
-### [1.12.1](https://github.com/googleapis/python-aiplatform/compare/v1.12.0...v1.12.1) (2022-04-20)
+## [1.12.1](https://github.com/googleapis/python-aiplatform/compare/v1.12.0...v1.12.1) (2022-04-20)
 
 
 ### Features