Update Arch install instructions

One should not use `-Syu` when installing packages, because system
updates should be separate from installing individual packages.

Also, mentioning that Arch used C++17 is not worth it anymore since this
is the default C++ standard of ROOT anyway.

install/index.md

@@ -152,11 +152,9 @@ $ yum install root
 Arch's [ROOT package](https://www.archlinux.org/packages/community/x86_64/root){:target="\_blank"} can be installed with
 
 ```sh
-$ pacman -Syu root
+$ sudo pacman -S root
 ```
 
-The Arch package uses C++17.
-
 ### Gentoo
 
 The Gentoo package for ROOT is [sci-physics/root](https://packages.gentoo.org/packages/sci-physics/root){:target="\_blank"}.