Use 1.0-RC1 instead of 1.0-rc1

As Weston mentioned,
this follows the WordPress convention.

amp.php

@@ -5,7 +5,7 @@
  * Plugin URI: https://github.com/automattic/amp-wp
  * Author: WordPress.com VIP, XWP, Google, and contributors
  * Author URI: https://github.com/Automattic/amp-wp/graphs/contributors
- * Version: 1.0-rc1
+ * Version: 1.0-RC1
  * Text Domain: amp
  * Domain Path: /languages/
  * License: GPLv2 or later
@@ -49,7 +49,7 @@ function _amp_print_composer_install_admin_notice() {
 
 define( 'AMP__FILE__', __FILE__ );
 define( 'AMP__DIR__', dirname( __FILE__ ) );
-define( 'AMP__VERSION', '1.0-rc1' );
+define( 'AMP__VERSION', '1.0-RC1' );
 
 require_once AMP__DIR__ . '/includes/class-amp-autoloader.php';
 AMP_Autoloader::register();