Fixed typo in ErrorHandler

`$this` -> `$self`

framework/yii/base/ErrorHandler.php

@@ -153,7 +153,7 @@ class ErrorHandler extends Component
 			// method/function call
 			$self = $this;
 			$html = preg_replace_callback('/^(.*)\(\)$/', function ($matches) use ($self) {
-				return '<a href="http://yiiframework.com/doc/api/2.0/' . $this->htmlEncode($matches[1]) . '" target="_blank">' .
+				return '<a href="http://yiiframework.com/doc/api/2.0/' . $self->htmlEncode($matches[1]) . '" target="_blank">' .
 					$self->htmlEncode($matches[1]) . '</a>()';
 			}, $code);
 		}