Change Area Code -> Zip Code (#119)

## Changes

<!-- What changes did you make? -->

- "Area Code" --> "Zip Code"

## Testing

<!-- How did you confirm your changes worked? -->

- n/a

## Confirmation of Change

<!-- Upload a screenshot, if possible. Otherwise, please provide
instructions on how to see the change. -->

<img width="827" alt="image"
src="https://github.com/TritonSE/USHS-Housing-Portal/assets/24444266/be1a04c1-0065-44a8-b4a6-4b8456eee6e6">

frontend/src/components/ListingFormComponents.tsx

@@ -537,7 +537,7 @@ export function ListingFormComponents(props: ListingFormComponentsProps) {
         </TextContainer>
         <TextContainer>
           <Textbox
-            elementName="Area Code"
+            elementName="Zip Code"
             requiredField={true}
             name="zip"
             value={areaCode}