abi: README.md: Update symbol list documentation

Recommend using 'build_abi.sh --update-symbol-list' instead of
extract_symbol directly as that abstracts away some of the complexity.

Bug: 158655966
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: Ib13c0fd4982523b91ae837d864efca09ce3445a8
1 file changed