Correct a comment in backdrop_settings_initialize()

[avpaderno]

backdrop_settings_initialize() contains the following comment.

// HTTP_HOST can be modified by a visitor, but we already sanitized it
// in backdrop_settings_initialize().

$_SERVER['HTTP_HOST'] is sanitized in backdrop_environment_initialize(), not backdrop_settings_initialize(). The correct comment would be:

// HTTP_HOST can be modified by a visitor, but we already sanitized it in
// backdrop_environment_initialize().