doc/go1.19: move the description of the runtime.GOROOT change from 'cmd/go' to 'runtime'
Even though the change in the behavior of 'runtime.GOROOT' was
not actually due to a change in the runtime package proper, I
suspect that users who notice it will look for the release note
in that section, not the 'cmd/go' section.
Fixes #51461.
Change-Id: I271752968d4152a7fdf3e170537e3072bf87ce86
Reviewed-on: https://go-review.googlesource.com/c/go/+/400814 Reviewed-by: Ian Lance Taylor <iant@google.com>