net/http: fix typo in docstring

s/activitiy/activity

Change-Id: Ib2bbc929b38b1993000da57daed2d795f4a93997
Reviewed-on: https://go-review.googlesource.com/18131
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/server.go

@@ -478,7 +478,7 @@ type readResult struct {
 // connReader is the io.Reader wrapper used by *conn. It combines a
 // selectively-activated io.LimitedReader (to bound request header
 // read sizes) with support for selectively keeping an io.Reader.Read
-// call blocked in a background goroutine to wait for activitiy and
+// call blocked in a background goroutine to wait for activity and
 // trigger a CloseNotifier channel.
 type connReader struct {
 	r      io.Reader