Fix the \since version for QSharedPointer::create().

It is not relevant that it has been part of the class since Qt 4.6. It has
always been internal until now. The release where it became public API (5.1)
is what is relevant.

Change-Id: Ib740f3ed6df190884a94fb2c11dd74cd7edb7b1a
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/corelib/tools/qsharedpointer.cpp

@@ -606,7 +606,7 @@
 
 /*!
     \fn QSharedPointer<T> QSharedPointer::create()
-    \since 4.6
+    \since 5.1
 
     Creates a QSharedPointer object and allocates a new item of type \tt T. The
     QSharedPointer internals and the object are allocated in one single memory