docs/ohos-build: Fix possessive grammar mistake

I have observed this in several places,
Without it's still understandable but it requiered me an extra effort.

This makes me think I think there are standards to check if
"Simple English" is used, this could help community adoption of project.

Forwarded: https://git.ostc-eu.org/OSTC/OHOS/meta-ohos/-/merge_requests/233


Signed-off-by: Philippe Coval <philippe.coval@astrolabe.coop>