minor #10435 Improved the article about session flashes (javiereguiluz)

This PR was submitted for the 4.1 branch but it was merged into the 3.4 branch instead (closes #10435).

Discussion
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Improved the article about session flashes

As discussed here symfony/demo#859  Not sure about the branch though.

Commits
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e6ce2f4 Improved the article about session flashes

Author: javiereguiluz

session/avoid_session_start.rst

@@ -29,7 +29,7 @@ access the flash messages:
 
 .. code-block:: html+twig
 
-    {% if app.request.hasPreviousSession %}
+    {% if app.request.method == 'POST' %}
         {% for message in app.flashes('notice') %}
             <div class="flash-notice">
                 {{ message }}