net/http: adjust wording on ServeMux godoc
When reading through the docs, I found that the sentence flows better with
the `to` present.
Change-Id: I51c21fa33c7a13748c0814dd87c76b9a8d64e2ec
GitHub-Last-Rev:
0642bd7185343ba562c7057071d4689ead7a378e
GitHub-Pull-Request: golang/go#63587
Reviewed-on: https://go-review.googlesource.com/c/go/+/535935
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>