Revert "revert pointless placeholder changes"

This reverts commit 694335cf3b.

app/src/frontend/building/data-containers/age.tsx

@@ -116,8 +116,8 @@ const AgeView: React.FunctionComponent<CategoryViewProps> = (props) => {
                 copy={props.copy}
                 onChange={props.onChange}
                 tooltip={dataFields.date_source.tooltip}
-                placeholder=""
                 options={dataFields.date_source.items}
+                placeholder={dataFields.date_source.example}
                 />
             <MultiDataEntry
                 title={dataFields.date_link.title}

app/src/frontend/config/data-fields-config.ts

@@ -294,6 +294,7 @@ export const dataFields = { /* eslint-disable @typescript-eslint/camelcase */
             "Other website",
             "Other"
         ],
+        example: "",
     },
     date_source_detail: {
         category: Category.Age,