net/http: fix some awkward wording in Request.WithContext docs

Change-Id: Ib37db42c7f1fd6aa55f70fd2d65d56bb2ae6d26a
Reviewed-on: https://go-review.googlesource.com/c/go/+/211098Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/net/http/request.go

@@ -350,8 +350,8 @@ func (r *Request) Context() context.Context {
 // sending the request, and reading the response headers and body.
 //
 // To create a new request with a context, use NewRequestWithContext.
-// To change the context of a request (such as an incoming) you then
-// also want to modify to send back out, use Request.Clone. Between
+// To change the context of a request, such as an incoming request you
+// want to modify before sending back out, use Request.Clone. Between
 // those two uses, it's rare to need WithContext.
 func (r *Request) WithContext(ctx context.Context) *Request {
 	if ctx == nil {