Doc: fix QSystemSemaphore example

acquire() doesn't take arguments.

Change-Id: I16f0169c40433cc3cbfcb577bd8386d217cccb12
Task-number: QTBUG-40055
Reviewed-by: Oliver Wolff <oliver.wolff@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/corelib/doc/snippets/code/src_corelib_kernel_qsystemsemaphore.cpp

@@ -51,8 +51,9 @@ sem.release(2);              // resources available == 3
 
 //! [1]
 QSystemSemaphore sem("market", 5, QSystemSemaphore::Create);
-sem.acquire(5);           // acquire all 5 resources
-sem.release(5);           // release the 5 resources
+for (int i = 0; i < 5; ++i)  // acquire all 5 resources
+    sem.acquire();
+sem.release(5);              // release the 5 resources
 //! [1]