Update WindowsPipes.php

This file has made that my assetic had an error, the property assignment have an extra "$" that it mustn't be present. Please correct it
bug #11156 Update WindowsPipes.php (jgonzalezlopez)

This PR was merged into the 2.6-dev branch.

Discussion
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Update WindowsPipes.php

This file has made that my assetic had an error, the property assignment have an extra "$" that it mustn't be present. Please correct it

Commits
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7bb98e2 Update WindowsPipes.php