log/slog: fix broken link to AnyValue in comment

log/slog: fix broken link to AnyValue in comment

Change-Id: Ida52a2bf6a415017942bf9ccd74a1ea9ed02bc46
Reviewed-on: https://go-review.googlesource.com/c/go/+/505535
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>