fix syntax error

lib/preview/libreoffice-cl.php

@@ -29,7 +29,7 @@ if (extension_loaded('imagick')) {
 			chdir(get_temp_dir());
 
 			$exec = $this->cmd . ' --headless -convert-to pdf ' . escapeshellarg($abspath);
-			exec($exec)
+			exec($exec);
 			
 			//create imagick object from pdf
 			try{