doc: fix non-existent config name

doc: fix non-existent config name

Replace non-existent `branch.<name>.fetch` to `remote.<repository>.fetch`, in
the first example in `git-fetch` doc, which was introduced in
d504f6975d (modernize fetch/merge/pull examples, 2009-10-21).

Rename placeholder `<name>` to `<repository>`, to be consistent with all other
uses in git docs, except that `git-config.txt` uses `remote.<name>.fetch` in
its "Variables" section.

Also add missing monospace markups.

Signed-off-by: Yukai Chou <muzimuzhi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>