x/website: ref/mod: go get is module aware but doesn't support `-mod` flag

[goto1134]

The (documentation)[https://go.dev/ref/mod#build-commands] says that go get is module-aware and supports -mod flag, but in fact it is not. (see #60104)

Please, fix the documentation.

[gopherbot]

Change https://go.dev/cl/494235 mentions this issue: _content/ref: remove go get from build commands