net/http,doc: use HTTP status code constants where applicable

There are a few places where the integer value is used.
Use the equivalent constants to aid with readability.

Change-Id: I023b1dbe605340544c056d0e0d9d6d5a7d7d0edc
GitHub-Last-Rev: c1c90bcd25
GitHub-Pull-Request: golang/go#24123
Reviewed-on: https://go-review.googlesource.com/96984
Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/progs/error3.go

@@ -33,7 +33,7 @@ type appHandler func(http.ResponseWriter, *http.Request) error
 
 func (fn appHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if err := fn(w, r); err != nil {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 	}
 }