todo.txt: Remove note for byref function assignment syntax

It was solved by commits starting at b3f8da2b, see also the discussion at:
http://www.freebasic.net/forum/viewtopic.php?f=17&t=20805&start=45
parser: Simplify cOperator()
parser: Fix FB_ERRMSG_* bracket/brace confusion

FB_ERRMSG_EXPECTEDLBRACKET/FB_ERRMSG_EXPECTEDRBRACKET referred to
"Expected '{'" and "Expected '}'", renamed to *BRACE to match CHAR_* enum.

Added a real FB_ERRMSG_EXPECTEDRBRACKET ("Expected ']'").