Update one more README.md reference to HTTPS.

Author: lgarron

README.md

@@ -62,7 +62,7 @@ SecureHeaders::Configuration.default do |config|
     # directive values: these values will directly translate into source directives
     default_src: %w('none'),
     base_uri: %w('self'),
-    block_all_mixed_content: true, # see http://www.w3.org/TR/mixed-content/
+    block_all_mixed_content: true, # see https://www.w3.org/TR/mixed-content/
     child_src: %w('self'), # if child-src isn't supported, the value for frame-src will be set.
     connect_src: %w(wss:),
     font_src: %w('self' data:),