Apply suggestions from code review

Co-authored-by: Steven <steven@ceriously.com>

Author: ijjk

packages/next/lib/load-custom-routes.ts

@@ -109,7 +109,7 @@ function checkHeader(route: Header): string[] {
   if (!Array.isArray(route.headers)) {
     invalidParts.push('`headers` field must be an array')
   } else if (route.headers.length === 0) {
-    invalidParts.push('`headers` field can not be empty')
+    invalidParts.push('`headers` field cannot be empty')
   } else {
     for (const header of route.headers) {
       if (!header || typeof header !== 'object') {

test/integration/invalid-custom-routes/test/index.test.js

@@ -38,7 +38,7 @@ const runTests = () => {
     const stderr = await getStderr()
 
     expect(stderr).toContain(
-      '`headers` field can not be empty for route {"source":"/:path*"'
+      '`headers` field cannot be empty for route {"source":"/:path*"'
     )
   })