doc: typo fixtures

"handers" should be "handlers" in some comments.

Refs: eggjs#5243.

site/docs/core/error-handling.md

@@ -95,17 +95,17 @@ module.exports = {
  onerror: {
  all(err, ctx) {
  // Define an error handler for all type of Response.
- // Once config.all present, other type of error handers will be ignored.
+ // Once config.all present, other type of error handlers will be ignored.
  ctx.body = 'error';
  ctx.status = 500;
  },
  html(err, ctx) {
- // html hander
+ // html handler
  ctx.body = '<h3>error</h3>';
  ctx.status = 500;
  },
  json(err, ctx) {
- // json hander
+ // json handler
  ctx.body = { message: 'error' };
  ctx.status = 500;
  },