gpart(8): ms-basic-data is used for exFAT on GPT

Be explicit about it; the first mention of exFAT is for the MBR type 'ntfs',
and the reader must work back from there to the GPT type and infer that a
reference to MBR ntfs type means ms-basic-data is canonical for exFAT.

(It'd also be great if gpart had convenient aliases that did the right
thing for the partition scheme, such as 'ntfs' => ms-basic-data on GPT
schemes or 'exfat' => 'ntfs' in MBR schemes. The tool is also bad about
providing user-meaningful reasons for EINVAL failures.)