docs: improve protocol.handle file examples (#42111)

* docs: improve protocol.handle file examples

* chore: fix lint

docs/api/protocol.md

@@ -9,10 +9,14 @@ An example of implementing a protocol that has the same effect as the
 
 ```js
 const { app, protocol, net } = require('electron')
+const path = require('node:path')
+const url = require('node:url')
 
 app.whenReady().then(() => {
-  protocol.handle('atom', (request) =>
-    net.fetch('file://' + request.url.slice('atom://'.length)))
+  protocol.handle('atom', (request) => {
+    const filePath = request.url.slice('atom://'.length)
+    return net.fetch(url.pathToFileURL(path.join(__dirname, filePath)).toString())
+  })
 })
 ```
 
@@ -42,7 +46,7 @@ app.whenReady().then(() => {
 
   ses.protocol.handle('atom', (request) => {
     const filePath = request.url.slice('atom://'.length)
-    return net.fetch(url.pathToFileURL(path.join(__dirname, filePath)).toString())
+    return net.fetch(url.pathToFileURL(path.resolve(__dirname, filePath)).toString())
   })
 
   const mainWindow = new BrowserWindow({ webPreferences: { partition } })