Fixed Typo in String Helper function

alnum spelled as alunum in random_string function documentation comment.
diff --git a/system/helpers/string_helper.php b/system/helpers/string_helper.php
index 12f818f..777acba 100644
--- a/system/helpers/string_helper.php
+++ b/system/helpers/string_helper.php
@@ -188,7 +188,7 @@
 	 *
 	 * Useful for generating passwords or hashes.
 	 *
-	 * @param	string	type of random string.  basic, alpha, alunum, numeric, nozero, unique, md5, encrypt and sha1
+	 * @param	string	type of random string.  basic, alpha, alnum, numeric, nozero, unique, md5, encrypt and sha1
 	 * @param	int	number of characters
 	 * @return	string
 	 */
@@ -294,4 +294,4 @@
 }
 
 /* End of file string_helper.php */
-/* Location: ./system/helpers/string_helper.php */
\ No newline at end of file
+/* Location: ./system/helpers/string_helper.php */