fix(ligatures): avoid invalid prettify-symbols-alist

Otherwise, `prettify-symbols-alist` will be set to a list whose first
element is the mode name, a symbol. That makes `prettify-symbols-alist`
an invalid alist, so when `prettify-symbols-mode` is enabled, there's a
type error in `prettify-symbols--make-keywords`.

Amend: c07f359d64

modules/ui/ligatures/config.el

@@ -112,7 +112,7 @@ isn't disabled in `+ligatures-extras-in-modes'."
         (((+ligatures--enable-p +ligatures-extras-in-modes))
          (symbols
           (if-let ((symbols (assq major-mode +ligatures-extra-alist)))
-              symbols
+              (cdr symbols)
             (cl-loop for (mode . symbols) in +ligatures-extra-alist
                      if (derived-mode-p mode)
                      return symbols))))