README.md: Update Fedora instructions
README.md: Update Fedora instructions

yum is outdated, we should instruct to use dnf instead.

Signed-off-by: Guilherme Paes <895ede20f271502d47e722126b280ac3a4bb391c@intel.com>

brunobottazzini/soletta-dev-app,solettaproject/soletta-dev-app,solettaproject/soletta-dev-app,brunobottazzini/soletta-dev-app,brunobottazzini/soletta-dev-app,solettaproject/soletta-dev-app