Fixed inconsistent headers

source/standards/diagrams-as-code.html.md.erb

@@ -24,17 +24,17 @@ Diagrams are a form of documentation, and some teams already use a '[docs as cod
 * version history
 * consistency
 
-### Diagrams as code tools
+## Diagrams as code tools
 
 Assuming you already [use GitHub](https://gds-way.digital.cabinet-office.gov.uk/standards/source-code/use-github.html) then you can [use Mermaid in Markdown files](https://github.blog/developer-skills/github/include-diagrams-markdown-files-mermaid/).
 
 The [Mermaid Live Editor](https://mermaid.live/) is a good way of trying out Mermaid, and seeing diagrams rendered from code in real time.
 
-### Open source diagrams as code
+## Open source diagrams as code
 
 Consider making the code for your diagrams open in the same way as [your other source code](https://gds-way.digital.cabinet-office.gov.uk/standards/source-code/index.html#publish-open-source-code).
 
-### Sequence diagrams as code
+## Sequence diagrams as code
 
 A [sequence diagram](https://en.wikipedia.org/wiki/Sequence_diagram) is used to show how different entities in a system interact with each other and can be a useful way of designing new functionality or documenting existing behaviour.