Typo in lparser.c

[Gabriel Bertilson]

In the comment before the function check_conflict, "begin used" should
be "being used".

/*
** check whether, in an assignment to an upvalue/local variable, the
** upvalue/local variable is begin used in a previous assignment to a
** table. If so, save original upvalue/local value in a safe place and
** use this safe copy in the previous assignment.
*/

— Gabriel