Fix fish config for thefuck on Linux

fish/config.fish

@@ -38,7 +38,7 @@ set --universal fish_greeting
 # https://github.com/nvbn/thefuck
 # =======
 
-if which -s thefuck
+if which thefuck > /dev/null
   eval (thefuck --alias | tr '\n' ';')
 end