fix spelling error

change "zope" to "Zope"

docs/zope2book/SearchingZCatalog.rst

@@ -1599,7 +1599,7 @@ purpose searching.  Traditionally, 'PrincipiaSearchSource'
 would include the text in an object's title, it's body, and
 anywhere else you'd want to be able to search. 
 
-For example, if you downloaded a zope product that managed
+For example, if you downloaded a Zope product that managed
 our zoo, and it had an Animal type that you could add to your
 site, this animal type would probably expose a 
 PrincipiaSearchSource that looked something like this::