smtp: add missing periods in comments

Merge from net/smtp/smtp.go

Obtained from:	b2faff18ce28edad98303d2c3134dec1331fd7b5

smtp.go

@@ -455,7 +455,7 @@ func (c *Client) Quit() error {
 	return c.Text.Close()
 }
 
-// validateLine checks to see if a line has CR or LF as per RFC 5321
+// validateLine checks to see if a line has CR or LF as per RFC 5321.
 func validateLine(line string) error {
 	if strings.ContainsAny(line, "\n\r") {
 		return errors.New("smtp: A line must not contain CR or LF")