Informative spec: slight changes to the home page

[agarny]

• "The official normative specification document [can be] download[ed] from:"
• We mention "See more" blocks, but they really do show as "See more+" blocks. So, might want to refer to them as such? (Having said all of that, only the "See more" part is clickable.)
• The home page mentions "© Copyright 2019-2020, CellML Editors and Contributors Revision 47b71ad.":
  • The CellML 2.0 specification was released on 17 April 2020, so the copyright should start in 2020, not 2019.
  • It would be nice if the revision information was clickable.