docs: fix CookieStore creation in doc.go (#206)

doc.go

@@ -31,7 +31,7 @@ Let's start with an example that shows the sessions API in a nutshell:
 	// alongside your code. Ensure your key is sufficiently random - i.e. use Go's
 	// crypto/rand or securecookie.GenerateRandomKey(32) and persist the result.
 	// Ensure SESSION_KEY exists in the environment, or sessions will fail.
-	var store = sessions.NewCookieStore(os.Getenv("SESSION_KEY"))
+	var store = sessions.NewCookieStore([]byte(os.Getenv("SESSION_KEY")))
 
 	func MyHandler(w http.ResponseWriter, r *http.Request) {
 		// Get a session. Get() always returns a session, even if empty.