[doc] QSet: fix return type of insert()

It's an iterator, not a const_iterator. Let QDoc figure out the correct one.

Change-Id: I7ddd1568adbf811b801c170794465ba14ceed05e
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/tools/qset.qdoc

@@ -530,7 +530,7 @@
 */
 
 /*!
-    \fn QSet::const_iterator QSet::insert(const T &value)
+    \fn QSet::insert(const T &value)
 
     Inserts item \a value into the set, if \a value isn't already
     in the set, and returns an iterator pointing at the inserted