set fo+=n

From :help fo-table:

n       When formatting text, recognize numbered lists.  This actually uses
        the 'formatlistpat' option, thus any kind of list can be used.  The
        indent of the text after the number is used for the next line.  The
        default is to find a number, optionally followed by '.', ':', ')',
        ']' or '}'.  Note that 'autoindent' must be set too.  Doesn't work
        well together with "2".                    

                1. the first item
                2. the second item