Fixes KafkaConsumer examples in README.rst

[yassinecharjami]

in the README.rst files KafkaConsumer take as argument the topic which is not correct, i corrected the two first examples from:

from kafka import KafkaConsumer
consumer = KafkaConsumer('my_favorite_topic')
for msg in consumer:
... print (msg)

join a consumer group for dynamic partition assignment and offset commits

from kafka import KafkaConsumer
consumer = KafkaConsumer('my_favorite_topic', group_id='my_favorite_group')
for msg in consumer:
... print (msg)

to:

from kafka import KafkaConsumer
consumer = KafkaConsumer(bootstrap_servers='localhost:1234')
consumer.subscribe(['my_favourite_topic'])
for msg in consumer:
... print (msg)

join a consumer group for dynamic partition assignment and offset commits

from kafka import KafkaConsumer
consumer = KafkaConsumer(bootstrap_servers='localhost:1234', group_id='my_favorite_group')
consumer.subscribe(['my_favourite_topic'])
for msg in consumer:
... print (msg)

Thanks

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This change is