Update themes.md

[fabioquarantini]

Status value is "active" and not "1". See "class-wp-rest-themes-controller.php" (Ln 91):

if ( isset( $registered['status'], $request['status'] ) && in_array( 'active', $request['status'], true ) ) {
$active_theme = wp_get_theme();
$active_theme = $this->prepare_item_for_response( $active_theme, $request );
$themes[] = $this->prepare_response_for_collection( $active_theme );
}

[kadamwhite]

Thanks for this PR. The /reference/ folder is auto-generated from the schema, so we'll need to look at why the value came through this way, rather than simply correcting the generated file -- otherwise this will be wiped out the next time the reference documentation is updated.