Fix incorrect comment in Polycurve_2_iostream.h

[LOTUS9679]

Summary of Changes

Fixed an incorrect comment in Arrangement_on_surface_2/include/CGAL/Arr_geometry_traits/IO/Polycurve_2_iostream.h (Line 36). The comment previously stated it exported "points", but the code actually exports number_of_subcurves(). I have updated the comment to correctly state "subcurves".

Release Management

• Affected package(s): Arrangement_on_surface_2
• Issue(s) solved (if any): fix #6257
• Feature/Small Feature (if any):
• Link to compiled documentation (obligatory for small feature):
• License and copyright ownership: I confirm I am the author of these changes.