website: Fix a stray broken link to 0.8 -> 0.9 backend upgrade doc
website: Fix a stray broken link to 0.8 -> 0.9 backend upgrade doc

I missed this earlier because my link checker was collapsing the two links from
this page to that page into a single report.

For posterity's sake, I'm linking directly to the markdown file in GitHub at an
appropriate tag version. I do not expect anyone to ever click this link again,
though.

apparentlymart/terraform,apparentlymart/terraform,ewbankkit/terraform,hashicorp/terraform,pdecat/terraform,hashicorp/terraform,apparentlymart/terraform,pdecat/terraform,ewbankkit/terraform,ewbankkit/terraform,rnaveiras/terraform,apparentlymart/terraform,rnaveiras/terraform,apparentlymart/terraform,tommynsong/terraform,tommynsong/terraform,hashicorp/terraform,rnaveiras/terraform,ewbankkit/terraform,tommynsong/terraform,pdecat/terraform,pdecat/terraform