unit_of_interest.rst: fix minor formatting issues

Author: pmderodat

doc/gnatcov/units_of_interest.rst

@@ -204,7 +204,7 @@ and then executing::
   gnatcov <command> --units=@units.list
 
 Conveying *subprograms* of interest
-*********************************
+***********************************
 
 |gcv| enables even finer grain control through the use of ``--subprograms``
 switch, which restricts coverage analysis to the specified list of subprograms