fix: update correct path to use when exporting 404 page (#5470)

When exporting error page, next defaults it to 404/index.html which is not recognized as a default 404 page.

This should fix https://github.com/zeit/next.js/issues/5035

packages/next/export/index.js

@@ -38,7 +38,7 @@ export default async function (dir, options, configuration) {
     }
 
     if (page === '/_error') {
-      defaultPathMap['/404'] = { page }
+      defaultPathMap['/404.html'] = { page }
       continue
     }
 

test/integration/export/test/ssr.js

@@ -53,5 +53,10 @@ export default function (context) {
       const html = await renderViaHTTP(context.port, '/404')
       expect(html).toMatch(/404/)
     })
+
+    it('should export 404.html instead of 404/index.html', async () => {
+      const html = await renderViaHTTP(context.port, '/404.html')
+      expect(html).toMatch(/404/)
+    })
   })
 }