t4014: "no-add-headers" is actually called "no-add-header"

t4014: "no-add-headers" is actually called "no-add-header"

Since c426003 (format-patch: add --no-cc, --no-to, and
--no-add-headers, 2010-03-07) the tests have checked for an option
called --no-add-headers introduced by letting the user negate
--add-header.

However, the parseopt machinery does not automatically pluralize
anything, so it is in fact called --no-add-header.

Since the option never worked, is not documented anywhere, and
implementing an actual --no-add-headers would lead to silly code
complications, we just adapt the test to the code.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>