docs: Fix a few typos

[timgates42]

There are small typos in:

• dev_tools/docker/cluster.rst
• docs/examples/sequence_file.rst
• pydoop/app/submit.py
• pydoop/avrolib.py
• src/libhdfs/jni_helper.h

Fixes:

• Should read bootstrap rather than boostrap.
• Should read zookeeper rather than zookeper.
• Should read singletons rather than singletones.
• Should read output rather than ouput.
• Should read explicitly rather than explicitely.
• Should read estimate rather than extimate.

Semi-automated pull request generated by https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

[simleo]

Thanks!