[iDNS] Fix mermaid reference zone example in Overview (#21296)

Author: RebeccaTamachiro

src/content/docs/dns/internal-dns/index.mdx

@@ -86,13 +86,13 @@ flowchart LR
         accDescr: Diagram exemplifying Internal DNS zones and records relationship
         subgraph View 111 - London
 				subgraph Zone 601 - local
-				S[@ A 192.0.2.10]
-				T[ghi.example A 192.0.2.15]
+				S["@ A 192.0.2.10"]
+				T["ghi.example A 192.0.2.15"]
 				end
         subgraph Zone 600 - example.local
-				X[@ A 192.0.2.1]
-				Y[abc A 192.0.2.6]
-				Z[def A 192.0.2.9]
+				X["@ A 192.0.2.1"]
+				Y["abc A 192.0.2.6"]
+				Z["def A 192.0.2.9"]
 				end
 				end
 ```