pull doc: correct outdated description of an example

pull doc: correct outdated description of an example

Since f269048754 (fetch: opportunistically update tracking refs,
2013-05-11), the underlying `git fetch` in `git pull <remote> <branch>`
updates the configured remote-tracking branch for <branch>.

However, an example in the 'Examples' section of the `git pull`
documentation still states that this is not the case.

Correct the description of this example.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>