fix syntax from yum to dnf

[sumantro93]

fix syntax from yum to dnf

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[MikeMcQuaid]

Why is this desirable?

[esemeniuc]

@MikeMcQuaid yum is deprecated, dnf is the successor. See https://www.linux.com/training-tutorials/what-you-need-know-about-fedoras-switch-yum-dnf/

[sjackman]

replace dnf with yum for CentOS and RHEL

RHEL and 8 CentOS 8 have dnf, but version 7 still uses yum. Since yum works with all distributions, it's preferable in these instructions. CentOS 7 is EOL on 2024-June-30.

[sjackman]

Although it won't be merged, thank you for your PR all the same, Sumantro.