list: Fix docs for APPEND to show that elements are optional

list(APPEND) has been able to append nothing since commit a06dcdba
(Allow LIST(APPEND) command to append nothing, 2008-01-16) but the
documentation still used to imply that at least one argument is
required.

Source/cmListCommand.h

@@ -63,7 +63,7 @@ public:
       "  list(LENGTH <list> <output variable>)\n"
       "  list(GET <list> <element index> [<element index> ...]\n"
       "       <output variable>)\n"
-      "  list(APPEND <list> <element> [<element> ...])\n"
+      "  list(APPEND <list> [<element> ...])\n"
       "  list(FIND <list> <value> <output variable>)\n"
       "  list(INSERT <list> <element_index> <element> [<element> ...])\n"
       "  list(REMOVE_ITEM <list> <value> [<value> ...])\n"