docs: addException => addThrowable (php-debugbar#419)

ExceptionsCollector->addException() is deprecated since php-debugbar#281.

Author: caugner

docs/base_collectors.md

@@ -59,7 +59,7 @@ Display exceptions
     try {
         throw new Exception('foobar');
     } catch (Exception $e) {
-        $debugbar['exceptions']->addException($e);
+        $debugbar['exceptions']->addThrowable($e);
     }
 
 ## PDO