x/website: double "instead"s in go1.21.html

[willfaught]

In https://go.dev/doc/go1.21, under "Tools":

instead of attempting to build the code and instead printing errors about unresolved imports or syntax errors.

This would read better as "instead of attempting to build the code and then printing errors about unresolved imports or syntax errors."

[ianlancetaylor]

Thanks, sent https://go.dev/cl/517217.

[gopherbot]

Change https://go.dev/cl/517217 mentions this issue: _content/doc/go1.21: fix grammar error