menus: don't show M-6 in the help lines of any prompt

None of the editing keystrokes are shown in the menus -- they are
all "blind" keys.  This corrects a mistake in the previous commit.

src/global.c

@@ -805,7 +805,7 @@ void shortcut_init(void)
 
 	add_to_funcs(do_mark, MMAIN,
 		N_("Set Mark"), WITHORSANS(mark_gist), TOGETHER, VIEW);
-	add_to_funcs(copy_text, MMOST,
+	add_to_funcs(copy_text, MMAIN,
 		N_("Copy"), WITHORSANS(copy_gist), BLANKAFTER, VIEW);
 #endif