Avoid `let` in favor of Bash native syntax (ShellCheck advice)

99bottles.sh

         *-*)        n=3;; #don't care about more than 3
     esac
     case $word in
-        *seven*)    let $((n = n+1));;
+        *seven*)    ((n = n+1)) ;;
     esac
     echo "$n"
 }