Update username of NERDTree plugin (#1208)

README.md

@@ -207,7 +207,7 @@ call plug#end()
 
 ```vim
 " NERD tree will be loaded on the first invocation of NERDTreeToggle command
-Plug 'scrooloose/nerdtree', { 'on': 'NERDTreeToggle' }
+Plug 'preservim/nerdtree', { 'on': 'NERDTreeToggle' }
 
 " Multiple commands
 Plug 'junegunn/vim-github-dashboard', { 'on': ['GHDashboard', 'GHActivity'] }