Documentation: Update 'linux-2.6.git' -> 'linux.git'

Documentation: Update 'linux-2.6.git' -> 'linux.git'

The 3.x tree has been out for a while now.  The -2.6 repository name
survived the initial release [1], but kernel.org now only lists
'linux.git' (for aegl as well as torvalds) [2].

[1]: http://article.gmane.org/gmane.linux.kernel/1147422
  On 2011-05-30 01:47:57 GMT, Linus Torvalds wrote:
  > ... yes, that means that my git tree is still called
  > "linux-2.6.git" on kernel.org.
[2]: http://git.kernel.org/cgit/

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>