minor #7190 Fixed code example wrong prepend() method declaration (eko)

This PR was merged into the 2.7 branch.

Discussion
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Fixed code example wrong prepend() method declaration

I've fixed a wrong method declaration located on the following page: http://symfony.com/doc/current/components/dependency_injection/compilation.html#prepending-configuration-passed-to-the-extension

Thank you

Commits
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d8fb154 Fixed code example wrong prepend() method declaration

Author: xabbuh