Fix deprecated call

[timburgess]

This fixes a deprecation I was seeing with Bison 3.4.1, the current version that ships in Ubuntu

parser.y:32.1-36: warning: deprecated directive, use ‘%define api.parser.class { Parser }’ [-Wdeprecated]
   32 | %define parser_class_name { Parser }
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
parser.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]