fix: quote parenthesized Mermaid edge label to avoid parse error

Mermaid's pipe-style edge labels need quotes when the text contains
parentheses; node labels in brackets were already fine.

docs/adding-a-store.md

@@ -243,7 +243,7 @@ No component, no DOM, no async — just data in, data out.
 
 ```mermaid
 flowchart LR
-  View["View\n(template event)"] -->|dispatch(Msg)| Dispatch["store.dispatch"]
+  View["View\n(template event)"] -->|"dispatch(Msg)"| Dispatch["store.dispatch"]
   Dispatch --> Reduce["reduce(state, Msg)"]
   Reduce -->|"new state"| Signal["state signal .set()"]
   Signal -->|"state() read in template"| View