[Confluence] Fix docstring for two methods

[Hummus-Ful]

Hi,

I've noticed the same incomplete sentence in two docstrings under Confluence.attach_file() and Confluence.attach_content().

The current sentences are:

:param content_type: Specify the HTTP content type. The default is

Based on the methods, the sentences should be;

:param content_type: Specify the HTTP content type. The default is "application/binary"