go.dev/cmd/internal/site: fix tests on Windows

The site will never run on Windows, but keeping the tests passing
makes the x/website TryBots happy, and it is easy to do.

Fixes golang/go#46501.

Change-Id: Ib9ea07dc4164fbea38b7624c32a6f46d7bab6739
Reviewed-on: https://go-review.googlesource.com/c/website/+/324510
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Website-Publish: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/go.dev/cmd/internal/site/page.go b/go.dev/cmd/internal/site/page.go
index ed080be..02c9f97 100644
--- a/go.dev/cmd/internal/site/page.go
+++ b/go.dev/cmd/internal/site/page.go
@@ -9,6 +9,7 @@
 	"fmt"
 	"io/ioutil"
 	"path"
+	"path/filepath"
 	"strings"
 	"time"
 
@@ -32,6 +33,7 @@
 // loadPage loads the site's page from the given file.
 // It returns the page but also adds the page to site.pages and site.pagesByID.
 func (site *Site) loadPage(file string) (*page, error) {
+	file = filepath.ToSlash(file)
 	id := strings.TrimPrefix(file, "_content/")
 	if id == "index.md" {
 		id = ""