doc/go1.21: fix HTML formatting

doc/go1.21: fix HTML formatting

Fix formatting mistakes in my previous CLs -- a missing code tag
and a broken comment tag.

Change-Id: I7f558f59b4e8fe9cb398d0093e5389b968d89eb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/496115
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>