README: update Arch package name (#663)

Looking at https://www.archlinux.org/packages/?q=numix ,
I don't see any `numix-themes` package, but a `numix-gtk-theme` one.

Author: ronjouch

README.md

@@ -7,7 +7,7 @@
 ### Distro Packages
 |Distro|Install Command/Links|
 |:----:|:----:|
-|![arch][arch]  ![antergos][antergos]|`sudo pacman -S numix-themes`|
+|![arch][arch]  ![antergos][antergos]|`sudo pacman -S numix-gtk-theme`|
 |![opensuse][opensuse]|[1 Click Install](http://software.opensuse.org/ymp/openSUSE:Factory/standard/numix-gtk-theme.ymp)  \|  [OBS Repo](http://software.opensuse.org/download.html?project=openSUSE%3AFactory&package=numix-gtk-theme)|
 |![fedora][fedora]|`sudo dnf install numix-gtk-theme`|
 |![debian][debian]  ![ubuntu][ubuntu]|`sudo apt install numix-gtk-theme`|