spl: Correct Kconfig help for TPL_BINMAN_SYMBOLS

Fix the help which should refer to TPL, not SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Suggested-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>