exfat: Improve wording of EXFAT_DEFAULT_IOCHARSET config option

  - Use consistent capitalization for "exFAT".
  - Fix grammar,
  - Split long sentence.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>