Fix formatting problem in the docs. (#435)

Sorry for the dumbest PR ever, but this tiny addition of a period changes the formatting of this sentence from a header to a regular paragraph in godoc.

client.go

@@ -140,7 +140,7 @@ var nilDialer = *DefaultDialer
 // Use the response.Header to get the selected subprotocol
 // (Sec-WebSocket-Protocol) and cookies (Set-Cookie).
 //
-// The context will be used in the request and in the Dialer
+// The context will be used in the request and in the Dialer.
 //
 // If the WebSocket handshake fails, ErrBadHandshake is returned along with a
 // non-nil *http.Response so that callers can handle redirects, authentication,