Specify in PHPDoc the return type of Configuration::getConfigTreeBuilder

[kaznovac]

Issue #, if available:
Resolves #88

Description of changes:
Added the return type of TreeBuilder to the Configuration::getConfigTreeBuilder to avoid the following Deprecation notice:

Method "Symfony\Component\Config\Definition\ConfigurationInterface::getConfigTreeBuilder()" might add "TreeBuilder" as a native return type declaration in the future. Do the same in implementation "Aws\Symfony\DependencyInjection\Configuration" now to avoid errors or add an explicit @return annotation to suppress this message.

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[RanVaknin]

Hi there,

It seems like this was already taken care of previously

aws-sdk-php-symfony/src/DependencyInjection/Configuration.php

Line 11 in 6ee6fc4

public function getConfigTreeBuilder(): TreeBuilder

now getConfigTreeBuilder explicitly returns TreeBuilder

Thanks,
Ran~