dbus-send.1: use bold and italic type consistently

dbus-send.1: use bold and italic type consistently

According to man-pages(7)

* bold is for literal text, the name of the thing being documented,
  or the name of another man page

* italic is for replaceable text, usually in all-caps

* normal type (in the SYNOPSIS) is for special syntax like the []
  indicating optional things

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=14005
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Lennart Poettering <lennart@poettering.net>