[mlir][openacc] NFC Fix acc.databounds description incorrect bullet

A dash used in description is an automatic bullet in documentation
if it is the first character of the line. The dash used in the
particular location being fixed was intended to be a separator.
Thus move it to previous line to fix incorrect bullet-ization.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D148968