docs: fix changelog header to consistent size (#175)

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Changelog
 
-### [1.3.1](https://github.com/googleapis/python-network-connectivity/compare/v1.3.0...v1.3.1) (2022-03-05)
+## [1.3.1](https://github.com/googleapis/python-network-connectivity/compare/v1.3.0...v1.3.1) (2022-03-05)
 
 
 ### Bug Fixes
@@ -28,14 +28,14 @@
 * add autogenerated code snippets ([bb6f7ae](https://github.com/googleapis/python-network-connectivity/commit/bb6f7ae110aa15f035b2a512e3f78fadfef564f9))
 * Update comments to reflect that spokes can now be created with data transfer disabled ([bb6f7ae](https://github.com/googleapis/python-network-connectivity/commit/bb6f7ae110aa15f035b2a512e3f78fadfef564f9))
 
-### [1.2.2](https://www.github.com/googleapis/python-network-connectivity/compare/v1.2.1...v1.2.2) (2021-11-11)
+## [1.2.2](https://www.github.com/googleapis/python-network-connectivity/compare/v1.2.1...v1.2.2) (2021-11-11)
 
 
 ### Bug Fixes
 
 * Mark API fields as required which were already required on the backend ([#107](https://www.github.com/googleapis/python-network-connectivity/issues/107)) ([17b9d5f](https://www.github.com/googleapis/python-network-connectivity/commit/17b9d5fb6f23494b759504ed56ef80e95b960620))
 
-### [1.2.1](https://www.github.com/googleapis/python-network-connectivity/compare/v1.2.0...v1.2.1) (2021-11-01)
+## [1.2.1](https://www.github.com/googleapis/python-network-connectivity/compare/v1.2.0...v1.2.1) (2021-11-01)
 
 
 ### Bug Fixes
@@ -62,7 +62,7 @@
 
 * add context manager support in client ([#88](https://www.github.com/googleapis/python-network-connectivity/issues/88)) ([8584d34](https://www.github.com/googleapis/python-network-connectivity/commit/8584d343d2c8709e3c467ee709ebc15fbbcbb970))
 
-### [1.0.1](https://www.github.com/googleapis/python-network-connectivity/compare/v1.0.0...v1.0.1) (2021-09-30)
+## [1.0.1](https://www.github.com/googleapis/python-network-connectivity/compare/v1.0.0...v1.0.1) (2021-09-30)
 
 
 ### Bug Fixes
@@ -76,7 +76,7 @@
 
 * bump release level to production/stable ([#65](https://www.github.com/googleapis/python-network-connectivity/issues/65)) ([e0876fb](https://www.github.com/googleapis/python-network-connectivity/commit/e0876fb2bcb03393a36c9ad1a65d5ca447259019))
 
-### [0.6.1](https://www.github.com/googleapis/python-network-connectivity/compare/v0.6.0...v0.6.1) (2021-09-24)
+## [0.6.1](https://www.github.com/googleapis/python-network-connectivity/compare/v0.6.0...v0.6.1) (2021-09-24)
 
 
 ### Bug Fixes