docs: Fix Python dynamic types example (#979)

Stumbled upon this broken example in the documentation. This:

- removes the `const` declaration 
- uses `snake_case` variable names
- imports the async client so that the example runs without
modifications

Related but not changed here: if copied as is (with the fixes), the
example will throw a `BamlValidationError` since the input `"some user
info"` doesn't contain the needed information. Replacing that with a
sentence like "Bill (16) plays soccer in Germany" will lead to a valid
output. Happy to pull up a patch for that as well if you want.

docs/docs/calling-baml/dynamic-types.mdx

@@ -177,20 +177,20 @@ Here we create a new `Hobbies` enum, and a new class called `Address`.
 
 ```python Python
 from baml_client.type_builder import TypeBuilder
-from baml_client import b
+from baml_client.async_client import b
 
 async def run():
   tb = TypeBuilder()
-  const hobbiesEnum = tb.add_enum('Hobbies')
-  hobbiesEnum.add_value('Soccer')
-  hobbiesEnum.add_value('Reading')
+  hobbies_enum = tb.add_enum("Hobbies")
+  hobbies_enum.add_value("Soccer")
+  hobbies_enum.add_value("Reading")
 
-  address_class = tb.add_class('Address')
-  address_class.add_property('street', tb.string())
+  address_class = tb.add_class("Address")
+  address_class.add_property("street", tb.string())
 
-  tb.User.add_property('hobby', hobbiesEnum.type().optional())
-  tb.User.add_property('address', addressClass.type().optional())
-  res = await b.DynamicUserCreator("some user info", { "tb": tb })
+  tb.User.add_property("hobby", hobbies_enum.type().optional())
+  tb.User.add_property("address", address_class.type().optional())
+  res = await b.DynamicUserCreator("some user info", {"tb": tb})
   # Now res might have the hobby property, which can be Soccer or Reading
   print(res)