Fix minor spelling error.

Author: arnt

src/Symfony/Component/Mailer/Tests/EnvelopeTest.php

@@ -83,7 +83,7 @@ public function testSenderFromHeadersWithoutFrom()
         $this->assertEquals($from, $e->getSender());
     }
 
-    public function testSenderFromHeadersWithMulitpleHeaders()
+    public function testSenderFromHeadersWithMultipleHeaders()
     {
         $headers = new Headers();
         $headers->addMailboxListHeader('From', [new Address('[email protected]', 'from'), '[email protected]']);