Update getting-started.md (jbesomi#88)

Fixing the custom pipeline example (custom pipeline missing from the `clean` parameters)

website/docs/getting-started.md

@@ -111,7 +111,7 @@ from texthero import preprocessing
 custom_pipeline = [preprocessing.fillna,
  preprocessing.lowercase,
  preprocessing.remove_whitespace]
-df['clean_text'] = hero.clean(df['text'])
+df['clean_text'] = hero.clean(df['text'], custom_pipeline)
 ```
 
 or alternatively