doc/go1.20: fix missing <code> tag

doc/go1.20: fix missing <code> tag

Change-Id: I9c6f9ec28dbe038ddc195310a32d97d5b2a28ef5
Reviewed-on: https://go-review.googlesource.com/c/go/+/453695
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>