scripts/get_abi: Fix wrong script file name in the help message

The help message of 'get_abi.pl' is mistakenly saying it's
'abi_book.pl'.  This commit fixes the wrong name in the help message.

Fixes: bbc249f2

 ("scripts: add an script to parse the ABI files")
Signed-off-by: SeongJae Park <sj@kernel.org>
Link: https://lore.kernel.org/r/20220419121636.290407-1-sj@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>