x/website: missing `"` in code snippet

[sko00o]

What version of Go are you using (go version)?

$ go version
# Doesn't matter.

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
# Doesn't matter.

What did you do?

Find typo when reading reference.

https://golang.org/ref/mod#environment-variables

What did you expect to see?

GOSUMDB="sum.golang.org+<publickey> https://sum.golang.org"

What did you see instead?

[ALTree]

cc @jayconrod

[gopherbot]

Change https://golang.org/cl/308429 mentions this issue: _content/doc: add missing quote in GOSUMDB documentation