Documentation/git-push.txt: correct configuration typo

Documentation/git-push.txt: correct configuration typo

In the EXAMPLES section, git-push(1) says that 'git push origin' pushes
the current branch to the value of the 'remote.origin.merge'
configuration.

This wording (which dates back to b2ed944af7 (push: switch default from
"matching" to "simple", 2013-01-04)) is incorrect. There is no such
configuration as 'remote.<name>.merge'. This likely was originally
intended to read "branch.<name>.merge" instead.

Indeed, when 'push.default' is 'simple' (which is the default value, and
is applicable in this scenario per "without additional configuration"),
setup_push_upstream() dies if the branch's local name does not match
'branch.<name>.merge'.

Correct this long-standing typo to resolve some recent confusion on the
intended behavior of this example.

Reported-by: Adam Sharafeddine <adam.shrfdn@gmail.com>
Reported-by: Fabien Terrani <terranifabien@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>