'main': Fix $flags_sans_argument for 'nice'.
'main': Fix $flags_sans_argument for 'nice'.

Follow-up to 2c15b0e996ad24ab0517a978ce43d1eab92086c7 ("Learn $flags_sans_arguments…").

joshuarubin/dotfiles,joshuarubin/dotfiles,phy1729/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,stuha/wbr-zsh,x-way/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,joshuarubin/dotfiles,P71/zsh-syntax-highlighting,joshuarubin/dotfiles