fixes the path and the install command for documentation.
fixes the path and the install command for documentation.

Fixes #958

mbudiu-vmw/p4c-clone,mbudiu-vmw/p4c-clone,p4lang/p4c,p4lang/p4c,mbudiu-vmw/p4c-clone,p4lang/p4c,hanw/p4c,p4lang/p4c,mbudiu-vmw/p4c-clone,hanw/p4c,p4lang/p4c,hanw/p4c,hanw/p4c