Fix grammar

docs/topics/practices.rst

@@ -111,7 +111,7 @@ reactor after ``MySpider`` has finished running.
     d.addBoth(lambda _: reactor.stop())
     reactor.run()  # the script will block here until the crawling is finished
 
-Same example but using a non-default reactor, is only necessary call ``install_reactor`` if you are using ``CrawlerRunner`` since ``CrawlerProcess`` already does automatically.
+Same example but using a non-default reactor, is only necessary call ``install_reactor`` if you are using ``CrawlerRunner`` since ``CrawlerProcess`` already this does automatically.
 
 .. code-block:: python