Incorrect directory layout in readme instructions

[ennerf]

The Readme instructions specify the directory layout:

hebi The C++ API and C binaries are stored in this directory (if cloning the repo directly, note that this directory is created by the download depends script.

However, the zip file contains a hebi-cpp folder that itself contains a hebi folder, so it's unclear which one to use. When using cmake, it downloads the API and creates a hebi-cpp folder.

[iamtesch]

Fixed.