Replace DOCKER_SDK to PROXY_WASM_CPP_SDK in readme

[110y]

After #1 , now we should use PROXY_WASM_CPP_SDK instead DOCKER_SDK, right....?

Actually by following current README, we cannot build wasm file like below since Makefiles use PROXY_WASM_CPP_SDK :

> docker run -v $PWD:/work -w /work  wasmsdk:v2 /build_wasm.sh
Adding directories to PATH:
PATH += /root/emsdk
PATH += /root/emsdk/upstream/emscripten
PATH += /root/emsdk/node/12.9.1_64bit/bin

Setting environment variables:
EMSDK = /root/emsdk
EM_CONFIG = /root/.emscripten
EMSDK_NODE = /root/emsdk/node/12.9.1_64bit/bin/node

make: Nothing to be done for 'all'.