docs: fix changelog header to consistent size (#767)

Co-authored-by: Anthonios Partheniou <partheniou@google.com>
Co-authored-by: Ilya Gurov <gurovilya9@gmail.com>

Author: 3 people

CHANGELOG.md

@@ -12,7 +12,7 @@
 
 * Add limitations docs on spanner features not supported by spanner-django library. ([#758](https://github.com/googleapis/python-spanner-django/issues/758)) ([66469bf](https://github.com/googleapis/python-spanner-django/commit/66469bf72dd57555d6729f8863d7d401b7f6613e))
 
-### [3.0.1](https://github.com/googleapis/python-spanner-django/compare/v3.0.0...v3.0.1) (2022-02-07)
+## [3.0.1](https://github.com/googleapis/python-spanner-django/compare/v3.0.0...v3.0.1) (2022-02-07)
 
 
 ### Bug Fixes