Fix URL scheme to match HTML page.

The HTML version of the license points to the HTTPS URL (564f4fcbb6/bedrock/mozorg/templates/mozorg/mpl/2.0/index.html (L135)) but the plaintext version is still using the legacy HTTP URL.

media/MPL/2.0/index.txt

@@ -357,7 +357,7 @@ Exhibit A - Source Code Form License Notice
 
   This Source Code Form is subject to the terms of the Mozilla Public
   License, v. 2.0. If a copy of the MPL was not distributed with this
-  file, You can obtain one at http://mozilla.org/MPL/2.0/.
+  file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
 If it is not possible or desirable to put the notice in a particular
 file, then You may include the notice in a location (such as a LICENSE