minor symfony#11046 Fix typo: if -> of (borNfreee)

This PR was merged into the 3.4 branch.

Discussion
----------

Fix typo: if -> of

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

7d91586 Fix typo: if -> of

Author: javiereguiluz

form/data_transformers.rst

@@ -252,7 +252,7 @@ that message with the ``invalid_message`` option (see below).
 Using the Transformer
 ~~~~~~~~~~~~~~~~~~~~~
 
-Next, you need to use the ``IssueToNumberTransformer`` object inside if ``TaskType``
+Next, you need to use the ``IssueToNumberTransformer`` object inside of ``TaskType``
 and add it to the ``issue`` field. No problem! Just add a ``__construct()`` method
 and type-hint the new class::