doc: fix unquoted use of "{type}"

doc: fix unquoted use of "{type}"

Curly braces open an "attribute" in AsciiDoc; if there's no
such attribute, strange things may happen. In this case, the
unquoted "{type}" causes AsciiDoc to omit an entire line of
text from the output. We can fix it by putting the whole
phrase inside literal backticks (which also lets us get rid
of ugly backslash escaping).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>