updated Arch linux install

[TimSousa1]

When I tried to install this package the command echo "source /usr/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh" >> ${ZDOTDIR:-$HOME}/.zshrc (present on the INSTALL.md file) didn't work since the file doesn't exist. The actual file is located on /usr/share/zsh/plugins/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh The only machine I have runs Arch Linux, and since I didn't want to assume that this is the case for most linux distributions I decided to just add the corresponding install instruction to the INSTALL.md file.