update documentation to recent pydispatcher import path change

docs/topics/extensions.rst

@@ -81,7 +81,7 @@ enabled.
 Let's take a look at the following example extension which just logs a message
 everytime a domain/spider is opened and closed::
 
-    from pydispatch import dispatcher
+    from scrapy.xlib.pydispatch import dispatcher
     from scrapy.core import signals
 
     class SpiderOpenCloseLogging(object):

docs/topics/item-pipeline.rst

@@ -66,7 +66,7 @@ that were already processed. Let say that our items has an unique id, but our
 spider returns multiples items with the same id::
 
 
-    from pydispatch import dispatcher
+    from scrapy.xlib.pydispatch import dispatcher
     from scrapy.core import signals
     from scrapy.core.exceptions import DropItem
 

docs/topics/webconsole.rst

@@ -58,7 +58,7 @@ Example web console extension
 Here's an example of a simple web console extension that just displays a "Hello
 world!" text::
 
-    from pydispatch import dispatcher
+    from scrapy.xlib.pydispatch import dispatcher
     from scrapy.management.web import webconsole_discover_module
 
     class HelloWorldConsole(object):