minor #13570 Update symfony_server.rst (seb-jean)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead.

Discussion
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Update symfony_server.rst

Remove quotes else there is a message :
WARNING the current dir requires PHP "xxx" (.php-version from current dir: $PATH\.php-version), but this version is not available

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Commits
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c7c8b44 Update symfony_server.rst

Author: javiereguiluz

setup/symfony_server.rst

@@ -110,10 +110,10 @@ root directory:
     $ cd my-project/
 
     # use a specific PHP version
-    $ echo "7.2" > .php-version
+    $ echo 7.2 > .php-version
 
     # use any PHP 7.x version available
-    $ echo "7" > .php-version
+    $ echo 7 > .php-version
 
 .. tip::