manual.texi Correct error in DBI NULLABLE description.

@findex DBI->@{NULLABLE@}

@findex NULLABLE DBI method

@item NULLABLE

Returns a reference to an array of boolean values; for each element of

the array, a value of TRUE indicates that this

column may contain @code{NULL} values.

Returns a reference to an array of values that indicate whether columns may

contain @code{NULL} values.

The possible values for each array element are 0 or the empty string if the

column cannot be @code{NULL}, 1 if it can, and 2 if the column's @code{NULL}

status is unknown.

Example:

@example

$null_possible = $sth->@{NULLABLE@};