Use right variable name to avoid zero-length array

This fixes the typographic error that always assigned an undefined
array to `$names`, which made `$list_size` = 0 and triggered a
divide by zero error on the next line.

plugins/emoji/emoji.plugin.zsh

@@ -255,7 +255,7 @@ function random_emoji() {
   if [[ -z "$group" || "$group" == "all" ]]; then
   	names=(${(k)emoji})
   else
-  	names=(${=emoji_groups[$group_name]})
+	names=(${=emoji_groups[$group]})
   fi
   local list_size=$#names
   local random_index=$(( ( RANDOM % $list_size ) + 1 ))