Site-www: 'Write HTTP Clients...` still uses `pub serve` instead of `webdev serve`

[pet-ro]

Page URL: https://www.dartlang.org/tutorials/dart-vm/httpserver
Page source: https://github.com/dart-lang/site-www/tree/master/src/_tutorials/dart-vm/httpserver.md

Still mentions pub serve to launch the webserver from the top directory of the app,
instead of webdev serve.

pubspec.yaml needs a change to

````pubspec.yaml
name: httpserver
description: Sample HTTP clients and servers

homepage: https://www.dartlang.org/docs/tutorials/httpserver/

dependencies:
http_server: ^0.9.6

dev_dependencies:
build_runner: '>=0.8.10 <0.11.0'
build_web_compilers: '>=0.3.6 <0.5.0'
test: ^1.3.0
````

[kwalrath]

I'll check for this in all pages.

[kwalrath]

I've updated the linked-to code: dart-lang/dart-tutorials-samples#46