t/README: fix multi-prerequisite example

t/README: fix multi-prerequisite example

With the broken quoting the test wouldn't even parse correctly, but
there's also the '==' instead of POSIX '=' (of the shells I tested,
busybox ash, bash and ksh (93 and OpenBSD) accept '==', dash and zsh do
not), and 'print 2' from Python 2 days.

(I assume the test failing due to 3 != 4 is intentional or immaterial.)

Fixes: 93a572461386 ("test-lib: Add support for multiple test prerequisites")
Signed-off-by: Štěpán Němec <stepnem@smrk.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>