[5.3] Fix type for custom form validators

[HLeithner]

Summary of Changes

Use the correct javascript method name.

Testing Instructions

This is not used in Joomla Core.

Simple test:

1. Open a form like article and save it. Should work.
2. Open a form like article and remove a required argument and save it. Should fail.

Custom Code:

Add a custom validator in the article override form or any other override like the login module in frontend.

Something like this:

This validator will fail:

            document.formvalidator.customValidators['validatorname'] = {
                exec: function() {
                    return false;
                }
            };

This validator will succeed:

            document.formvalidator.customValidators['validatorname'] = {
                exec: function() {
                    return true;
                }
            };

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[Fedik]

I have tested this item ✅ successfully on d7e181b

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[QuyTon]

I have tested this item ✅ successfully on d7e181b

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45650.}

[QuyTon]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45650.}

[LadySolveig]

Thanks for the fix and to all the testers!