fix typo on upgrade page (#249)

Author: Chris Cho

source/upgrade.txt

@@ -100,7 +100,7 @@ Version 4.0 Breaking Changes
 - The default BSON representation of ``java.util.UUID`` value was changed
   from ``JAVA_LEGACY`` to ``UNSPECIFIED``. Applications that store or retrieve
   UUID values must explicitly specify which representation to use. You can
-  specify the representation in the ``uidRepresentation`` property of
+  specify the representation in the ``uuidRepresentation`` property of
   ``MongoClientSettings``.
 - The connection pool no longer restricts the number of wait queue threads
   or asynchronous tasks that require a connection to MongoDB. The