minor #5897 Fixed some wrong line number references in doctrine.rst (DigNative)

This PR was merged into the 2.3 branch.

Discussion
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Fixed some wrong line number references in doctrine.rst

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none

Some line numbers for the code example in section "Persisting Objects to the Database" were wrong.

Commits
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50274c4 Fixed some wrong line numbers in doctrine.rst

Author: xabbuh

book/doctrine.rst

@@ -534,10 +534,10 @@ Take a look at the previous example in more detail:
   responsible for handling the process of persisting and fetching objects
   to and from the database.
 
-* **line 16** The ``persist()`` method tells Doctrine to "manage" the ``$product``
+* **line 17** The ``persist()`` method tells Doctrine to "manage" the ``$product``
   object. This does not actually cause a query to be made to the database (yet).
 
-* **line 17** When the ``flush()`` method is called, Doctrine looks through
+* **line 18** When the ``flush()`` method is called, Doctrine looks through
   all of the objects that it's managing to see if they need to be persisted
   to the database. In this example, the ``$product`` object has not been
   persisted yet, so the entity manager executes an ``INSERT`` query and a