[docs] Fix dart-mode-hook

Author: ericdallo

README.md

@@ -31,7 +31,7 @@ The following has a example to setup `lsp-dart`.
   (package-refresh-contents)
   (mapc #'package-install package-selected-packages))
 
-(add-hook 'dart-mode 'lsp)
+(add-hook 'dart-mode-hook 'lsp)
 
 (setq gc-cons-threshold (* 100 1024 1024)
       read-process-output-max (* 1024 1024)