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/state.html

@@ -17,11 +17,11 @@
     <h1>State diagram demos</h1>
     <h2>Very simple showing change from State1 to State2</h2>
     <pre class="mermaid">
----
-title: Very simple diagram
----
-		stateDiagram
-		  accTitle: This is the accessible title
+    ---
+    title: Very simple diagram
+    ---
+    stateDiagram
+      accTitle: This is the accessible title
       accDescr:This is an accessible description
       State1 --> State2
     </pre>
@@ -47,13 +47,13 @@ title: Very simple diagram
       </code>
     </p>
     <pre class="mermaid">
----
-title: Very simple diagram
----
-		stateDiagram
+    ---
+    title: Very simple diagram
+    ---
+    stateDiagram
       direction TB
 
-		  accTitle: This is the accessible title
+      accTitle: This is the accessible title
       accDescr: This is an accessible description
 
       classDef notMoving fill:white