doc: changed order of invocations in example.

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc

Description of change

When you call req.end() before you add .on listeners you get an Error that you can't call .on on undefined.