PYTH12 — Python mapping issue: ValueFactory

The ValueFactory mapping is slightly unclear. I suggest the following
revised text for the third paragraph of section 1.3.10:

For a given value type, the ValueFactory maps to a class instance
with a _call_ method taking no arguments. When it is called, it
returns a new instance of the value type. Initialiser operations of
the value type map to methods of the factory object. The registry
for value factories can be accessed using the standard ORB
operations register_value_factory, unregister_value_factory, and
lookup_value_factory. For value types without operations, a default
factory is registered automatically.

Accept the proposed changes