_content/blog/intro-generics: add import for "constraints"
_content/blog/intro-generics: add import for "constraints"

In the first code sample that uses the `constraints` package, add an explicit
import for it; otherwise this code sample will not work in Go 1.18

Change-Id: I4e9823b34128cc068a77947223ac8e71c7fdf722
Reviewed-on: https://go-review.googlesource.com/c/website/+/395334
Run-TryBot: Eli Bendersky‎ <e83e4a9813852caf164372e4b6bd506b7819d6ab@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Eli Bendersky <e83e4a9813852caf164372e4b6bd506b7819d6ab@google.com>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website