fix(examples): forgotten mermaid diagram example (tldraw#8916)

quick "oops" here, forgot to add a mermaid diagram as an example

### Change type

- [ ] `bugfix`
- [x] `improvement`
- [ ] `feature`
- [ ] `api`
- [ ] `other`

Author: kaneel

apps/examples/src/examples/use-cases/mermaid-pasting/README.md

@@ -18,3 +18,14 @@ Supported diagrams:
 - State
 - Sequence
 - Mindmap
+
+Example:
+
+```
+graph TD
+    A[Start] --> B{Is it correct LALA?}
+    B -- Yes --> C[Display Diagram]
+    B -- No --> D[Edit Code]
+    D --> B
+    C --> E[End]
+```