Spelling fixes - `wether` to `whether`

Interestingly `wether` means a castrated ram in old English
diff --git a/system/helpers/download_helper.php b/system/helpers/download_helper.php
index 5efbc49..09c4de5 100644
--- a/system/helpers/download_helper.php
+++ b/system/helpers/download_helper.php
@@ -46,7 +46,7 @@
 	 *
 	 * @param	string	filename
 	 * @param	mixed	the data to be downloaded
-	 * @param	bool	wether to try and send the actual file MIME type
+	 * @param	bool	whether to try and send the actual file MIME type
 	 * @return	void
 	 */
 	function force_download($filename = '', $data = '', $set_mime = FALSE)