Correct php-http/httplug version

[reyostallenberg]

Correct php-http/httplug version form "^1.0" to "^2.0" to reflect an installable Your Final composer.json

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	-
Documentation	-
License	MIT

What's in this PR?

When using the example on this page you'll get:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - php-http/mock-client[dev-master, 1.5.0] require php-http/httplug ^2.0 -> found php-http/httplug[dev-master, v2.0.0, 2.1.0, 2.2.0, 2.x-dev (alias of dev-master)] but it conflicts with your root composer.json require (^1.0).
    - php-http/mock-client 1.x-dev is an alias of php-http/mock-client dev-master and thus requires it to be installed too.
    - Root composer.json requires psr/http-client-implementation ^1.0 -> satisfiable by php-http/mock-client[dev-master, 1.5.0, 1.x-dev (alias of dev-master)].

Why?

This makes the example directly usable

[dbu]

thanks!