minor #5704 Fix typo in translation.rst (zatikbalazs)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #5704).

Discussion
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Fix typo in translation.rst

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
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5637d6b Fix typo in translation.rst

book/translation.rst

@@ -590,7 +590,7 @@ use somewhere in your application::
         public $name;
     }
 
-Add constraints though any of the supported methods. Set the message option to the
+Add constraints through any of the supported methods. Set the message option to the
 translation source text. For example, to guarantee that the ``$name`` property is
 not empty, add the following: