fix guide with field_with_error proc example

[asanghi]

The "Active Record Validations and Callbacks" guide section 9.3 "Customizing the Error Messages HTML" seems to indicate that

ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  errors = Array(instance.error_message).join(',')
  %(#{html_tag}<span class="validation-error">&nbsp;#{errors}</span>).html_safe
end

Might be a suitable option to customizing a field with error. But in the most common case this would wrap around the label as well as inputs like text_field helpers, leading to the error beings displayed twice.

The suggested fix to the guide ensures that the error is not displayed in case of labels. I'm sorry, but I just cooked up the fix in a couple of minutes so feel free if you think the solution could be better.

[rafaelfranca]

Seems good. @carlosantoniodasilva WDYT?

[asanghi]

Can this be backported to the current guides as well if found suitable? or would I need to send another pull request?

[rafaelfranca]

We can backport

[gaurish]

👍

[carlosantoniodasilva]

Yeah, seems fine. Thanks @asanghi :)