Fix grammer in comment regarding attachment security

Commit f017e81 had a typo within one of
the new code comment blocks.

file_download.php

@@ -61,8 +61,8 @@
 # and the malicious HTML content would be rendered in the user's browser,
 # violating cross-domain security.
 if ( $f_show_inline ) {
-	# Disable errors for form_security_validate as we need to first need to
-	# send HTTP headers prior to raising an error (the error handler within
+	# Disable errors for form_security_validate as we need to send HTTP
+	# headers prior to raising an error (the error handler within
 	# error_api.php doesn't check that headers have been sent, it just
 	# makes the assumption that they've been sent already).
 	if ( !@form_security_validate( 'file_show_inline' ) ) {