x/website: linux/riscv64 is missing from valid combinations list

[jfcg]

Doc site misses linux,riscv64 combo under GOOS,GOARCH combo list.
However go 1.15.5 can build with this combo and riscv64 is listed as a supported instruction set in introduction.

[ALTree]

Looks like the doc page in the main repo was updated here: https://go-review.googlesource.com/c/go/+/250997

But no corresponding CL was sent to the website repository:
https://github.com/golang/website/blob/c8f27351e764fa8954631d243dd24779108bd026/content/static/doc/install-source.html#L530

[gopherbot]

Change https://golang.org/cl/275495 mentions this issue: content/static/doc: list linux/riscv64 as a valid combination