[docs] Remove “Content-Type: application/x-www-form-urlencoded; charset” advice (issue 25576)

[rdmurray at bitdance.com]

http://bugs.python.org/review/25576/diff/15901/Doc/library/urllib.request.rst
File Doc/library/urllib.request.rst (right):

http://bugs.python.org/review/25576/diff/15901/Doc/library/urllib.request.rst#newcode1245
Doc/library/urllib.request.rst:1245: >>> with
urllib.request.urlopen("http://requestb.in/xrbl82xr", data) as f:
We lose something from the example no longer showing a use of
add_header.  Is there any other header we could add that would make
sense in context?  I don't think it is a big deal, though.

http://bugs.python.org/review/25576/