minor symfony#6432 fixed yaml config error (RickieL)

This PR was merged into the 3.0 branch.

Discussion
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fixed yaml config error

fixed the error of yaml config.
error info is "Unrecognized option "charset" under "doctrine"

Commits
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6c42309 fixed yaml config error

Author: xabbuh

Diff for: book/doctrine.rst

@@ -150,8 +150,8 @@ can automatically generate an empty ``test_project`` database for you:
 
             # app/config/config.yml
             doctrine:
-                charset: utf8mb4
                 dbal:
+                    charset: utf8mb4
                     default_table_options:
                         charset: utf8mb4
                         collate: utf8mb4_unicode_ci