Update install.md

[anigkus]

fixbenchmark1172,remove unused sigma_gn variable.

git submodule update --remote

third_party/benchmark from * (HEAD detached at bf585a2) to * (HEAD detached at e991355)

[uglide]

@anigkus Could you explain why this change should be added?

[anigkus]

@uglide

make snappy occurce error,unused sigma_gn variable,must update benchmark newest code.

cd 3rdparty/snappy
cmake -DHAVE_LIBLZO2=0 -DHAVE_LIBLZ4=0 && make