minor #13221 Fixed the Table of Contents of the performance article (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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Fixed the Table of Contents of the performance article

If you see the rendered page for Performance (https://symfony.com/doc/3.4/performance.html) you'll see that the TOC looks weird because of the location of some headers. This fixes it.

Commits
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3d82a61 Fixed the Table of Contents of the performance article

Author: javiereguiluz

performance.rst

@@ -11,19 +11,13 @@ performance checklists.
 Symfony Application Checklist
 -----------------------------
 
+These are the code and configuration changes that you can make in your Symfony
+application to improve its performance:
+
 #. :ref:`Install APCu Polyfill if your server uses APC <performance-install-apcu-polyfill>`
 #. :ref:`Enable APC Caching for the Autoloader <performance-autoloader-apc-cache>`
 #. :ref:`Use Bootstrap Files <performance-use-bootstrap-files>`
 
-Production Server Checklist
----------------------------
-
-#. :ref:`Use the OPcache byte code cache <performance-use-opcache>`
-#. :ref:`Configure OPcache for maximum performance <performance-configure-opcache>`
-#. :ref:`Don't check PHP files timestamps <performance-dont-check-timestamps>`
-#. :ref:`Configure the PHP realpath Cache <performance-configure-realpath-cache>`
-#. :ref:`Optimize Composer Autoloader <performance-optimize-composer-autoloader>`
-
 .. _performance-install-apcu-polyfill:
 
 Install APCu Polyfill if your Server Uses APC
@@ -108,6 +102,18 @@ rebuilt after updating the vendor libraries via the ``composer install`` command
   course, if this feature is disabled in the byte code cache (e.g.
   ``apc.stat=0`` in APC), there is no longer a reason to use a bootstrap file.
 
+Production Server Checklist
+---------------------------
+
+These are the changes that you can make in your production server to improve
+performance when running Symfony applications:
+
+#. :ref:`Use the OPcache byte code cache <performance-use-opcache>`
+#. :ref:`Configure OPcache for maximum performance <performance-configure-opcache>`
+#. :ref:`Don't check PHP files timestamps <performance-dont-check-timestamps>`
+#. :ref:`Configure the PHP realpath Cache <performance-configure-realpath-cache>`
+#. :ref:`Optimize Composer Autoloader <performance-optimize-composer-autoloader>`
+
 .. _performance-use-opcache:
 
 Use the OPcache Byte Code Cache