Optimized installation instructions

[lykahb]

The plugin zsh-completions has a workaround to avoid double initialization of completions zsh-users/zsh-completions#603. Its installation manual recommends setting fpath instead of adding this to the list of plugins.

fpath+=${ZSH_CUSTOM:-${ZSH:-~/.oh-my-zsh}/custom}/plugins/zsh-completions/src

It may be worth for nix-zsh-completions to do the same. I verified that this installation method also works.