net/smtp: fix spelling mistake

Change-Id: I3141076c0db7b70ea408c941a857ed1f82dcb2df
Reviewed-on: https://go-review.googlesource.com/73557Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>

src/net/smtp/smtp.go

@@ -399,7 +399,7 @@ func (c *Client) Reset() error {
 }
 
 // Noop sends the NOOP command to the server. It does nothing but check
-// that the connaction to the server is okay.
+// that the connection to the server is okay.
 func (c *Client) Noop() error {
 	if err := c.hello(); err != nil {
 		return err