Civil procedure — Relief from judgment — Excusable neglect — Trial court erred in denying plaintiff’s rule 1.540(b) motion seeking relief from dismissal of case after plaintiff missed court’s deadline to file demand for arbitration — Motion provided a reasonable and credible explanation for the missed deadline based on secretarial error where counsel submitted affidavit from counsel’s legal assistant wherein the legal assistant explained that it was her first time handling an arbitration case and, based on a misreading of a form, the legal assistant believed her initial request for mediation also served as a demand for arbitration — Trial court erroneously faulted counsel for entrusting his legal assistant to complete demand for arbitration form where form did not require an attorney’s signature and required only basic case information