doc/articles: format error string as per go style

First letter of an error string should not be capitalized, as prescribed in the [wiki](https://github.com/golang/go/wiki/Errors).

Change-Id: Iea1413f19b5240d3bef79f216094d210b54bdb62
GitHub-Last-Rev: d8e167107122b603c4f647d722537668ad1c680d
GitHub-Pull-Request: golang/go#35203
Reviewed-on: https://go-review.googlesource.com/c/go/+/203797Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/articles/wiki/final-noclosure.go

@@ -90,7 +90,7 @@ func getTitle(w http.ResponseWriter, r *http.Request) (string, error) {
 	m := validPath.FindStringSubmatch(r.URL.Path)
 	if m == nil {
 		http.NotFound(w, r)
-		return "", errors.New("Invalid Page Title")
+		return "", errors.New("invalid Page Title")
 	}
 	return m[2], nil // The title is the second subexpression.
 }