docs(demos): fix indentation of YAML front-matter

Mermaid diagrams that have YAML front-matter can now be indented in
HTML code, see commit:
5cfa9196 (fix: support parsing indented mmd YAML from HTML, 2022-11-27)

Some diagrams previously had a mix of tabs/spaces for indentation.
In order for `dedent` to work, these diagrams had to be converted to
using a consistent indentation.

demos/classchart.html

@@ -17,10 +17,10 @@
     <h1>Class diagram demos</h1>
 
     <pre class="mermaid">
----
-title: Demo Class Diagram
----
-		classDiagram
+    ---
+    title: Demo Class Diagram
+    ---
+    classDiagram
       accTitle: Demo Class Diagram
       accDescr: This class diagram show the abstract Animal class, and 3 classes that inherit from it: Duck, Fish, and Zebra.