Updated Custom campaigns (markdown)

Custom-campaigns.md

@@ -109,6 +109,14 @@ Incorrect:
         - VFA-113
 ```
 
+Incorrect:
+```
+    - primary: SEAD
+      secondary: air-to-ground
+    - aircraft:
+        - VFA-113
+```
+
 Correct:
 ```
     - primary: SEAD