Fixed #20890 -- Added missing import in class-based view docs.

Thanks André Augusto.

docs/topics/class-based-views/intro.txt

@@ -198,6 +198,7 @@ A similar class-based view might look like::
 
     from django.http import HttpResponseRedirect
     from django.shortcuts import render
+    from django.views.generic.base import View
 
     from .forms import MyForm