Correct QThread::idealThreadCount() docs

QThread::idealThreadCount() now returns 1 if the number
of CPU cores could not be detected.

Change-Id: I60b75c46fbfa2891c28e71fed65589e2ce5a5c17
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qthread.cpp

@@ -291,7 +291,7 @@ QThreadPrivate::~QThreadPrivate()
     \fn int QThread::idealThreadCount()
 
     Returns the ideal number of threads that can be run on the system. This is done querying
-    the number of processor cores, both real and logical, in the system. This function returns -1
+    the number of processor cores, both real and logical, in the system. This function returns 1
     if the number of processor cores could not be detected.
 */