Fix example usage of BaseFormatter.for_type_by_name.

I propose a small documentation fix for this excellent feature of
IPython.

The current example doesn't work because the type module and type name
are lumped into one string. They must be given as separate strings.

See docstring:

```
Signature: BaseFormatter.for_type_by_name(self, type_module, type_name, func=None)
```

For extra clarity, I have added a submodule to this made-up example
module so that the pattern is clear.

docs/source/config/integrating.rst

@@ -85,7 +85,7 b' Formatters for third-party types'
 
 The user can also register formatters for types without modifying the class::
 
-    from bar import Foo
+    from bar.baz import Foo
 
     def foo_html(obj):
         return '<marquee>Foo object %s</marquee>' % obj.name
@@ -94,7 +94,7 b' The user can also register formatters for types without modifying the class::'
     html_formatter.for_type(Foo, foo_html)
 
     # Or register a type without importing it - this does the same as above:
-    html_formatter.for_type_by_name('bar.Foo', foo_html)
+    html_formatter.for_type_by_name('bar.baz', 'Foo', foo_html)
 
 Custom exception tracebacks
 ===========================