doc: coverage: fix coverage report generation instruction

Fix build step to enable CONFIG_COVERAGE_DUMP, otherwise
we won't get coverage data dump required for next step.

Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>