improve code for CustomNSError example (#8660)

Defining the function in the property would create a new function every time the `errorUserInfo` property is accessed.

src/platform-includes/capture-error/apple.mdx

@@ -60,17 +60,17 @@ enum MyCustomError: Error {
 
 extension MyCustomError: CustomNSError {
     var errorUserInfo: [String : Any] {
-        func getDebugDescription() -> String {
-            switch self {
-            case .indexOutOfBounds:
-                return  "indexOutOfBounds"
-            case .enumeratingWhileMutating:
-                return "enumeratingWhileMutating"
-            }
-        }
-
         return [NSDebugDescriptionErrorKey: getDebugDescription()]
     }
+
+    private func getDebugDescription() -> String {
+        switch self {
+        case .indexOutOfBounds:
+            return "indexOutOfBounds"
+        case .enumeratingWhileMutating:
+            return "enumeratingWhileMutating"
+        }
+    }
 }
 ```