minor #6858 Fix documentation about addColumn() method (befresh-mweimerskirch)

This PR was merged into the 4.x branch.

Discussion
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Fix documentation about addColumn() method

This is a minor documentation fix.
When using addColumn() with an integer, it is transformed to col-md-N and not col-N.

See https://github.com/EasyCorp/EasyAdminBundle/blob/4.x/src/Field/FormField.php#L139

Commits
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cab02f0 Fix documentation about addColumn() method

Author: javiereguiluz

doc/fields.rst

@@ -388,8 +388,8 @@ The arguments of the ``addColumn()`` method are:
 
 * ``$cols``: (type: ``int|string``) the width of the column defined as any value
   compatible with the `Bootstrap grid system`_  (e.g. ``'col-6'``, ``'col-md-6 col-xl-4'``,
-  etc.). Integer values are transformed like this: N -> 'col-N' (e.g. ``8`` is
-  transformed to ``col-8``);
+  etc.). Integer values are transformed like this: N -> 'col-md-N' (e.g. ``8`` is
+  transformed to ``col-md-8``);
 * ``$label``: (type: ``TranslatableInterface|string|false|null``) an optional title
   that is displayed at the top of the column. If you pass ``false``, ``null``
   or an empty string, no title is displayed. You can also pass ``string`` and