[docs] 'codecs' module docs improvements (issue 19548)

[vadmium+py at gmail.com]

https://bugs.python.org/review/19548/diff/13445/Doc/library/codecs.rst
File Doc/library/codecs.rst (right):

https://bugs.python.org/review/19548/diff/13445/Doc/library/codecs.rst#newcode74
Doc/library/codecs.rst:74: .. data:: name
Looking at other files I guess the syntax here is meant to be ..
attribute::, not .. data::

https://bugs.python.org/review/19548/