Fix some line numbers in a code example

Author: javiereguiluz

migration.rst

@@ -289,15 +289,15 @@ could look something like this::
 
 There are 2 major deviations from the original file:
 
-Line 15
+Line 18
   First of all, ``$kernel`` is made globally available. This allows you to use
   Symfony features inside your existing application and gives access to
   services configured in our Symfony application. This helps you prepare your
   own code to work better within the Symfony application before you transition
   it over. For instance, by replacing outdated or redundant libraries with
   Symfony components.
 
-Line 38 - 47
+Line 41 - 50
   Instead of sending the Symfony response directly, a ``LegacyBridge`` is
   called to decide whether the legacy application should be booted and used to
   create the response instead.