doc/go1.19: fix a few links that were missing trailing slashes

doc/go1.19: fix a few links that were missing trailing slashes

They were part of a chain of three redirects. Now it's only two,
but that's #54081.

Change-Id: Ibfe6a17a372df61df40124892951b48577e4ced9
Reviewed-on: https://go-review.googlesource.com/c/go/+/419634
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>