Build step in readme does not build python binding

[wenxuanou]

In the original build step, the command cmake . --target build will do nothing anything after cmake . -B build, and will not build the python binding. Besides, cmake . -B build will create the build folder automatically and there is no need to manually create the folder.

The correct building commands should be:

git clone https://github.com/javedulu/ad-xolib.git
cd ad-xolib
git submodule update --init --recursive
cmake . -B build
cmake --build build

Please check.