CHANGELOG.md: The statement about the support situation of MRIv2.0 was changed.

HI,

I rewrite tha statment aboud the supporot situation of MRIv2.0

  For MRIv2.0 with patchlevel lower than 451, the function is
  succeedingly offered not using NativeExtension but using TracePointAPI which
  Ruby offers.

regards,