minor #11528 Update and rename from_flat_php_to_symfony2.rst to from_flat_php_to_s… (andreybolonin)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes #11528).

Discussion
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Update and rename from_flat_php_to_symfony2.rst to from_flat_php_to_s…

…ymfony.rst

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Commits
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80e44ee Update and rename from_flat_php_to_symfony2.rst to from_flat_php_to_symfony.rst

Author: javiereguiluz

introduction/from_flat_php_to_symfony2.rst renamed to introduction/from_flat_php_to_symfony.rst

@@ -1,7 +1,7 @@
 .. index::
    single: Symfony versus Flat PHP
 
-.. _symfony2-versus-flat-php:
+.. _symfony-versus-flat-php:
 
 Symfony versus Flat PHP
 =======================
@@ -523,7 +523,7 @@ allowing HTTP headers and content to be added via an object-oriented interface.
 And while the responses in this application are simple, this flexibility
 will pay dividends as your application grows.
 
-.. _the-sample-application-in-symfony2:
+.. _the-sample-application-in-symfony:
 
 The Sample Application in Symfony
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~