[Docs]The document of python function example is incorrect

[tuteng]

Describe the bug
There was an error in the example of using python function that causes the following exception to be thrown:

2019-11-11 09:48:43 +0000] [ERROR] log.py: Traceback (most recent call last):
[2019-11-11 09:48:43 +0000] [ERROR] log.py:   File "/pulsar/instances/python-instance/python_instance_main.py", line 211, in <module>
[2019-11-11 09:48:43 +0000] [ERROR] log.py: main()
[2019-11-11 09:48:43 +0000] [ERROR] log.py:   File "/pulsar/instances/python-[instance/python_instance_main.py", line 192, in main
[2019-11-11 09:48:43 +0000] [ERROR] log.py: pyinstance.run()
[2019-11-11 09:48:43 +0000] [ERROR] log.py:   File "/pulsar/instances/python-instance/python_instance.py", line 186, in run
[2019-11-11 09:48:43 +0000] [ERROR] log.py: self.input_serdes[topic] = serde_kclass()
[2019-11-11 09:48:43 +0000] [ERROR] log.py: TypeError
[2019-11-11 09:48:43 +0000] [ERROR] log.py: :
[2019-11-11 09:48:43 +0000] [ERROR] log.py: __init__() takes exactly 2 arguments (1 given)

Parameters should not appear in the initialization function of class TweetSerDe, messages should come from the topic and should not be assigned during initialization. the document example is wrong, and the correct example should be:

class Tweet(object):
    def __init__(self, username, tweet_content):
        self.username = username
        self.tweet_content = tweet_content


from pulsar import SerDe

class TweetSerDe(SerDe):
    def __init__(self):
        pass

    def serialize(self, input):
        return bytes("{0}|{1}".format(input.username, input.tweet_content))

    def deserialize(self, input_bytes):
        tweet_components = str(input_bytes).split('|')
        return Tweet(tweet_components[0], tweet_componentsp[1])

Reference: https://github.com/apache/pulsar/blob/master/pulsar-functions/python-examples/custom_object_function.py

Expected behavior
Fix document

Additional context
Use the correct example on the document

[candlerb]

Also,

    def __init__(self):
        pass

is redundant and can be omitted entirely.