doc: driver-model/platform.txt: fix a typo

The sentence is not proper English, so the use of "general" is now replaced with
"in general". "generally" was also considered, but "in general" is probably the
correct in this case.

Signed-off-by: Laszlo Papp <lpapp@kde.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>