fix(plugin-import-export): incorrect custom type on toCSVFunction changed to toCSV (#12796)

Type declaration extending `custom.['plugin-import-export']` was
incorrectly named `toCSVFunction` instead of `toCSV`. This changes the
type to match the correct property name `toCSV`.

packages/plugin-import-export/src/index.ts

@@ -91,7 +91,7 @@ export const importExportPlugin =
 declare module 'payload' {
   export interface FieldCustom {
     'plugin-import-export'?: {
-      toCSVFunction?: ToCSVFunction
+      toCSV?: ToCSVFunction
     }
   }
 }