Docs: OpenHarmony Quick Build - Grammatical error fix

* New sentence 'Once the image is built, you can run a Qemu X86-64 instance using the
  provided script wrapper as follows:' is added under Booting a Qemu
  X86-64 target with a OpenHarmony image, Fixes issue #77.

Signed-off-by: Arul Kumaran <arulkumaran.devarajan@huawei.com>