Comply with the XDG protocol in the README (#966)

README.md

@@ -59,8 +59,8 @@ $uri = 'https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
 ###### Unix, Linux
 
 ```sh
-curl -fLo ~/.local/share/nvim/site/autoload/plug.vim --create-dirs \
-    https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
+sh -c 'curl -fLo "${XDG_DATA_HOME:-$HOME/.local/share}"/nvim/site/autoload/plug.vim --create-dirs \
+       https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
 ```
 
 ###### Linux (Flatpak)
@@ -315,4 +315,3 @@ The commands with `!` suffix ensure that all steps are run unconditionally.
 ### License
 
 MIT
-