dev-manual: replace "on the contrary" with "by contrast"

The qualifier "By contrast" is more appropriate here, since that
phrase is used to introduce a point that is strikingly different from
what was just discussed, that being the substantial systemd.

(From yocto-docs rev: 3e73c7985dceb865e648f86ccd82aa0e7fc6668e)

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/init-manager.rst

@@ -44,7 +44,7 @@ therefore increasing the total system boot time. systemd also substantially
 increases system size because of its multiple components and the extra
 dependencies it pulls.
 
-On the contrary, BusyBox init is the simplest and the lightest solution and
+By contrast, BusyBox init is the simplest and the lightest solution and
 also comes with BusyBox mdev as device manager, a lighter replacement to
 :wikipedia:`udev <Udev>`, which SysVinit and systemd both use.