http: fix typo in http.Request documentation

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/2004044

src/pkg/http/request.go

@@ -71,7 +71,7 @@ type Request struct {
 	ProtoMajor int    // 1
 	ProtoMinor int    // 0
 
-	// A header mapping request lines to their values.
+	// A header is a mapping of request lines to their values.
 	// If the header says
 	//
 	//	accept-encoding: gzip, deflate