Fix modify_column() issues (#2020)
diff --git a/system/database/drivers/oci8/oci8_forge.php b/system/database/drivers/oci8/oci8_forge.php
index bd9d2df..a0efa95 100644
--- a/system/database/drivers/oci8/oci8_forge.php
+++ b/system/database/drivers/oci8/oci8_forge.php
@@ -105,7 +105,7 @@
 
 		$sql .= ' '.$alter_type.' ';
 		$sql .= (count($field) === 1)
-				? $fields[0]
+				? $field[0]
 				: '('.implode(',', $field).')';
 
 		// RENAME COLUMN must be executed after MODIFY