minor #9572 Corrected Bootstrap 4 custom forms example code (kriswillis)

This PR was submitted for the 4.0 branch but it was merged into the 3.4 branch instead (closes #9572).

Discussion
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Corrected Bootstrap 4 custom forms example code

Commits
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4681148 Corrected Bootstrap 4 custom forms example code

Author: javiereguiluz

form/bootstrap4.rst

@@ -97,8 +97,8 @@ and ``checkbox-custom`` respectively.
 
 .. code-block:: html+twig
 
-    {{ form_row(form.myRadio, {attr: {class: 'radio-custom'} }) }}
-    {{ form_row(form.myCheckbox, {attr: {class: 'checkbox-custom'} }) }}
+    {{ form_row(form.myRadio, {label_attr: {class: 'radio-custom'} }) }}
+    {{ form_row(form.myCheckbox, {label_attr: {class: 'checkbox-custom'} }) }}
 
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