changed let to type for better documentation and consistency with tutorial
changed let to type for better documentation and consistency with tutorial

hoheinzollern/FStar,hoheinzollern/FStar,FStarLang/FStar,FStarLang/FStar,A-Manning/FStar,A-Manning/FStar,hoheinzollern/FStar,A-Manning/FStar,hoheinzollern/FStar,FStarLang/FStar