[docs] Typo in class io.BufferedIOBase docs (issue 22671)

[storchaka at gmail.com]

https://bugs.python.org/review/22671/diff/13484/Lib/test/test_io.py
File Lib/test/test_io.py (right):

https://bugs.python.org/review/22671/diff/13484/Lib/test/test_io.py#newcode608
Lib/test/test_io.py:608: # Exercise the default RawIOBase.read() and
readall() implementations
May be create separate test for readall?

https://bugs.python.org/review/22671/diff/13484/Lib/test/test_io.py#newcode636
Lib/test/test_io.py:636: dict(method="readinto", avail=10, request=5,
result=5),
This is too verbose. Tuples would look cleaner. And testing code with
test["..."] looks untidy.

https://bugs.python.org/review/22671/diff/13484/Lib/test/test_io.py#newcode644
Lib/test/test_io.py:644: )
Does it make sense to add tests for request=0 or -1?

https://bugs.python.org/review/22671/diff/13484/Lib/test/test_io.py#newcode654
Lib/test/test_io.py:654: self.assertSequenceEqual(buffer[:size],
range(size))
Fill buffer before reading and test that buffer[size:] is not changed.

https://bugs.python.org/review/22671/