git-log.txt: fix typesetting of example "git-log -L" invocation

All surrounding examples are typeset as monospaced text. Follow suit.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>