Fix typo.

src/frame.c

@@ -2515,7 +2515,7 @@ For values specific to the separate minibuffer frame, see
  `minibuffer-frame-alist'.
 The `menu-bar-lines' element of the list controls whether new frames
  have menu bars; `menu-bar-mode' works by altering this element.
-Setting this variable does not affect exsiting frames, only new ones.  */);
+Setting this variable does not affect existing frames, only new ones.  */);
   Vdefault_frame_alist = Qnil;
 
   Qinhibit_default_face_x_resources