Fix new-error command (#44720)

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

plopfile.js

@@ -1,5 +1,7 @@
 module.exports = function (plop) {
-  plop.setHelper('toFileName', (str) => str.toLowerCase().replace(/ /g, '-'))
+  function getFileName(str) {
+    return str.toLowerCase().replace(/ /g, '-')
+  }
 
   plop.setGenerator('test', {
     description: 'Create a new test',
@@ -76,10 +78,11 @@ module.exports = function (plop) {
       },
     ],
     actions: function (data) {
+      const fileName = getFileName(data.urlPath)
       return [
         {
           type: 'add',
-          path: `errors/{{ toFileName name }}.md`,
+          path: `errors/${fileName}.md`,
           templateFile: `errors/template.txt`,
         },
         {
@@ -88,13 +91,13 @@ module.exports = function (plop) {
           transform(fileContents, data) {
             const manifestData = JSON.parse(fileContents)
             manifestData.routes[0].routes.push({
-              title: '{{ toFileName name }}',
-              path: `/errors/{{ toFileName name }}.md`,
+              title: fileName,
+              path: `/errors/${fileName}.md`,
             })
             return JSON.stringify(manifestData, null, 2)
           },
         },
-        `Url for the error: https://nextjs.org/docs/messages/{{ toFileName name }}`,
+        `Url for the error: https://nextjs.org/docs/messages/${fileName}`,
       ]
     },
   })