docs: update webContents.printToPDF() example for promisified API (electron#21550)

trop[bot] · zcbenz · commit 2dc42971ce8e · 2019-12-18T15:20:52.000+09:00
References: electron#16795 electron#17907
diff --git a/docs/api/web-contents.md b/docs/api/web-contents.md
@@ -1323,12 +1323,13 @@ win.loadURL('http://github.com')
 
 win.webContents.on('did-finish-load', () => {
   // Use default printing options
-  win.webContents.printToPDF({}, (error, data) => {
-    if (error) throw error
+  win.webContents.printToPDF({}).then(data => {
     fs.writeFile('/tmp/print.pdf', data, (error) => {
       if (error) throw error
       console.log('Write PDF successfully.')
     })
+  }).catch(error => {
+    console.log(error)
   })
 })
 ```
