Wrong path to doxygen built documentation in build_oai script

When you run "./build_oai --build-doxygen" The script indicates:

The documentation file is located here: $OPENAIR_DIR/$BUILD_DIR/build/html/index.html

This is wrong and should be:

The documentation file is located here: $OPENAIR_DIR/cmake_targets/ran_build/build/doc/html/index.html