-the official name is "OS X" and this change is only for
the documentation. Macros and API are untouched by this commit.

Change-Id: Ibdcfe2b8460685a6f8f895548288571dbb6e8a1d
Task-number: QTBUG-40759
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>