minor symfony#4034 Update internals.rst (redstar504)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes symfony#4034).

Discussion
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Update internals.rst

The proper idiom is "behind the scenes".

I also fixed the sentence structure.

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9966b90 Update internals.rst

Author: weaverryan

book/internals.rst

@@ -10,8 +10,8 @@ Symfony2 internals.
 
 .. note::
 
-    You need to read this section only if you want to understand how Symfony2
-    works behind the scene, or if you want to extend Symfony2.
+    You only need to read this section if you want to understand how Symfony2
+    works behind the scenes, or if you want to extend Symfony2.
 
 Overview
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