Update description of BUILD_COVERAGE option. [skip ci].
Update description of BUILD_COVERAGE option. [skip ci].

ennehekma/enne-stage,ennehekma/enne-stage,ennehekma/enne-stage,abhi-agrawal/ATOM_ADR,kartikkumar/cpp-project,abhi-agrawal/TestAtomProject,abhi-agrawal/ATOM_ADR,kartikkumar/cppbase,abhi-agrawal/TestAtomProject,kartikkumar/cpp-project