Update README.md

Hi,
fedora yum is obsolite log time ago...
https://www.2daygeek.com/comparison-difference-between-dnf-vs-yum/

Works for fedora 31:

[root@Galleon ~]# uname -a
Linux Galleon 5.6.13-200.fc31.x86_64 #1 SMP Thu May 14 23:26:14 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
[root@Galleon ~]#

README.md

@@ -25,7 +25,7 @@ $ cmake .. && make -j
 * Homebrew: `brew install spdlog`
 * MacPorts: `sudo port install spdlog`
 * FreeBSD:  `cd /usr/ports/devel/spdlog/ && make install clean`
-* Fedora: `yum install spdlog`
+* Fedora: `dnf install spdlog`
 * Gentoo: `emerge dev-libs/spdlog`
 * Arch Linux: `pacman -S spdlog`
 * vcpkg: `vcpkg install spdlog`