fix for project instructions

[kwk]

This is to avoid this warning with dnf install clang

Updating and loading repositories:
 Copr copr.fedorainfracloud.org/@fedora-llvm-team/llvm-snapshots external runtime dependency #1 - https_download_copr_fedorainfracloud_org_results_40fedora_llvm_team_llvm_compat_packages_distname_releasever_basearch                                                                                                                                                                                                                             100% | 903.0   B/s |   1.3 KiB |  00m02s
>>> Status code: 404 for https://download.copr.fedorainfracloud.org/results/%40fedora-llvm-team/llvm-compat-packages/$distname-41-x86_64/repodata/repomd.xml (IP: 2600:9000:2509:1000:4:bbc1:1840:93a1) - https://download.copr.fedorainfracloud.org/results/%40fedora-llvm-team/llvm-compat-packages/$distname-41-x86_64/repodata/repomd.xml - repomd.xml
>>> Status code: 404 for https://download.copr.fedorainfracloud.org/results/%40fedora-llvm-team/llvm-compat-packages/$distname-41-x86_64/repodata/repomd.xml (IP: 2600:9000:2509:1000:4:bbc1:1840:93a1) - https://download.copr.fedorainfracloud.org/results/%40fedora-llvm-team/llvm-compat-packages/$distname-41-x86_64/repodata/repomd.xml - repomd.xml
>>> Status code: 404 for https://download.copr.fedorainfracloud.org/results/%40fedora-llvm-team/llvm-compat-packages/$distname-41-x86_64/repodata/repomd.xml (IP: 2600:9000:2509:1000:4:bbc1:1840:93a1) - https://download.copr.fedorainfracloud.org/results/%40fedora-llvm-team/llvm-compat-packages/$distname-41-x86_64/repodata/repomd.xml - repomd.xml
>>> Status code: 404 for https://download.copr.fedorainfracloud.org/results/%40fedora-llvm-team/llvm-compat-packages/$distname-41-x86_64/repodata/repomd.xml (IP: 2600:9000:2509:1000:4:bbc1:1840:93a1) - https://download.copr.fedorainfracloud.org/results/%40fedora-llvm-team/llvm-compat-packages/$distname-41-x86_64/repodata/repomd.xml - repomd.xml
>>> Librepo error: Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried

Obviously $distname isn't properly replaced without this patch.

See also fedora-copr/copr#3387
And #670 where the issue was fixed by using dnf-3 instead of dnf.